I tried with 2.38 release branch (27339a3eb8f987eebae72b854af80256c1588ebd) with gcc 13 (e6ade7fb8a8595d7c5c2520c8ed497dbbe37ab3f) with the options that you seems to be using (--enable-stack-protector=no --enable-fortify-source=3) and I can't reproduce it. Any idea on how to easily reproduce it?

I've also tried with 2.38 release branch (cfe121910013a46e2477562282c56ae8062089aa) with gcc 13 (5d0d592c486c31bd9c26af0d1f0a7b6a3d68c22b) and I don't get a build fail.

If you manually run the gcc-compile-command producing strchrnul-c.os in <glibc-src>/string/ with -save-temps, the strchrnul-c.i file should have included <glibc-src>/string/strchrnul.c with __strchrnul_c instead of __strchrnul.

Then the internal symbol __GI___strchrnul should be __strchrnul_c (See your error-message from <glibc-src>/sysdeps/s390/strchrnul-c.c).
The weak_alias should be the IFUNC __strchrnul (See <glibc-src>/sysdeps/s390/strchrnul.c)

Sorry I am juggling a bit too many balls at the same time right now.

Since it's gentoo specific (apparently) it's not so urgent. => later
Not forgetting about this though (it makes our s390 stages fail).

So, after a lot of trying I found the culprit: --disable-multi-arch 

Without that, all builds fine. 

With that, I get the same failure with current master.

gcc version 13.2.1 20231216 (Gentoo 13.2.1_p20231216 p11) 
GNU assembler version 2.41.0 (s390-ibm-linux-gnu) using BFD version (Gentoo 2.41 p4) 2.41.0
Linux lgentoo3 6.0.9-gentoo #1 SMP Thu Nov 24 15:51:23 EST 2022 s390 8561 IBM GNU/Linux
master at ae49a7b29acc184b03c2a6bd6ac01b5e08efd54f

../glibc/configure --prefix=/usr --enable-fortify-source --disable-multi-arch

[...]

gcc ../sysdeps/s390/strchrnul-vx.S -c    -I../include -I/home/dilfridge/Gentoo/misc/glibc-build/string  -I/home/dilfridge/Gentoo/misc/glibc-build  -I../sysdeps/unix/sysv/linux/s390/s390-32  -I../sysdeps/unix/sysv/linux/s390/fpu  -I../sysdeps/s390/fpu  -I../sysdeps/unix/sysv/linux/s390  -I../sysdeps/s390/nptl  -I../sysdeps/ieee754/ldbl-64-128  -I../sysdeps/ieee754/ldbl-opt  -I../sysdeps/unix/sysv/linux/include -I../sysdeps/unix/sysv/linux  -I../sysdeps/nptl  -I../sysdeps/pthread  -I../sysdeps/gnu  -I../sysdeps/unix/inet  -I../sysdeps/unix/sysv  -I../sysdeps/unix  -I../sysdeps/posix  -I../sysdeps/s390/s390-32  -I../sysdeps/wordsize-32  -I../sysdeps/s390  -I../sysdeps/ieee754/ldbl-128  -I../sysdeps/ieee754/dbl-64  -I../sysdeps/ieee754/flt-32  -I../sysdeps/ieee754  -I../sysdeps/generic  -I.. -I../libio -I.  -D_LIBC_REENTRANT -include /home/dilfridge/Gentoo/misc/glibc-build/libc-modules.h -DMODULE_NAME=libc -include ../include/libc-symbols.h  -DPIC -DSHARED     -DTOP_NAMESPACE=glibc -DASSEMBLER  -g -Werror=undef -Wa,--noexecstack   -o /home/dilfridge/Gentoo/misc/glibc-build/string/strchrnul-vx.os -MD -MP -MF /home/dilfridge/Gentoo/misc/glibc-build/string/strchrnul-vx.os.dt -MT /home/dilfridge/Gentoo/misc/glibc-build/string/strchrnul-vx.os
gcc ../sysdeps/s390/strchrnul-c.c -c -std=gnu11 -fgnu89-inline  -g -O2 -Wall -Wwrite-strings -Wundef -Werror -fmerge-all-constants -frounding-math -fno-stack-protector -fno-common -Wp,-U_FORTIFY_SOURCE,-D_FORTIFY_SOURCE=3 -Wstrict-prototypes -Wold-style-definition -fmath-errno  -mlong-double-128  -fPIC     -ftls-model=initial-exec     -I../include -I/home/dilfridge/Gentoo/misc/glibc-build/string  -I/home/dilfridge/Gentoo/misc/glibc-build  -I../sysdeps/unix/sysv/linux/s390/s390-32  -I../sysdeps/unix/sysv/linux/s390/fpu  -I../sysdeps/s390/fpu  -I../sysdeps/unix/sysv/linux/s390  -I../sysdeps/s390/nptl  -I../sysdeps/ieee754/ldbl-64-128  -I../sysdeps/ieee754/ldbl-opt  -I../sysdeps/unix/sysv/linux/include -I../sysdeps/unix/sysv/linux  -I../sysdeps/nptl  -I../sysdeps/pthread  -I../sysdeps/gnu  -I../sysdeps/unix/inet  -I../sysdeps/unix/sysv  -I../sysdeps/unix  -I../sysdeps/posix  -I../sysdeps/s390/s390-32  -I../sysdeps/wordsize-32  -I../sysdeps/s390  -I../sysdeps/ieee754/ldbl-128  -I../sysdeps/ieee754/dbl-64  -I../sysdeps/ieee754/flt-32  -I../sysdeps/ieee754  -I../sysdeps/generic  -I.. -I../libio -I.  -D_LIBC_REENTRANT -include /home/dilfridge/Gentoo/misc/glibc-build/libc-modules.h -DMODULE_NAME=libc -include ../include/libc-symbols.h  -DPIC -DSHARED     -DTOP_NAMESPACE=glibc -o /home/dilfridge/Gentoo/misc/glibc-build/string/strchrnul-c.os -MD -MP -MF /home/dilfridge/Gentoo/misc/glibc-build/string/strchrnul-c.os.dt -MT /home/dilfridge/Gentoo/misc/glibc-build/string/strchrnul-c.os
gcc ../sysdeps/s390/strrchr-vx.S -c    -I../include -I/home/dilfridge/Gentoo/misc/glibc-build/string  -I/home/dilfridge/Gentoo/misc/glibc-build  -I../sysdeps/unix/sysv/linux/s390/s390-32  -I../sysdeps/unix/sysv/linux/s390/fpu  -I../sysdeps/s390/fpu  -I../sysdeps/unix/sysv/linux/s390  -I../sysdeps/s390/nptl  -I../sysdeps/ieee754/ldbl-64-128  -I../sysdeps/ieee754/ldbl-opt  -I../sysdeps/unix/sysv/linux/include -I../sysdeps/unix/sysv/linux  -I../sysdeps/nptl  -I../sysdeps/pthread  -I../sysdeps/gnu  -I../sysdeps/unix/inet  -I../sysdeps/unix/sysv  -I../sysdeps/unix  -I../sysdeps/posix  -I../sysdeps/s390/s390-32  -I../sysdeps/wordsize-32  -I../sysdeps/s390  -I../sysdeps/ieee754/ldbl-128  -I../sysdeps/ieee754/dbl-64  -I../sysdeps/ieee754/flt-32  -I../sysdeps/ieee754  -I../sysdeps/generic  -I.. -I../libio -I.  -D_LIBC_REENTRANT -include /home/dilfridge/Gentoo/misc/glibc-build/libc-modules.h -DMODULE_NAME=libc -include ../include/libc-symbols.h  -DPIC -DSHARED     -DTOP_NAMESPACE=glibc -DASSEMBLER  -g -Werror=undef -Wa,--noexecstack   -o /home/dilfridge/Gentoo/misc/glibc-build/string/strrchr-vx.os -MD -MP -MF /home/dilfridge/Gentoo/misc/glibc-build/string/strrchr-vx.os.dt -MT /home/dilfridge/Gentoo/misc/glibc-build/string/strrchr-vx.os
In file included from <command-line>:
../sysdeps/s390/strchrnul-c.c:28:49: error: ‘__strchrnul_c’ undeclared here (not in a function); did you mean ‘__strchrnul’?
   28 | __hidden_ver1 (__strchrnul_c, __GI___strchrnul, __strchrnul_c);
      |                                                 ^~~~~~~~~~~~~
./../include/libc-symbols.h:470:27: note: in definition of macro ‘__hidden_ver2’
  470 |   extern thread __typeof (name) __EI_##name \
      |                           ^~~~
../sysdeps/s390/strchrnul-c.c:28:1: note: in expansion of macro ‘__hidden_ver1’
   28 | __hidden_ver1 (__strchrnul_c, __GI___strchrnul, __strchrnul_c);
      | ^~~~~~~~~~~~~
./../include/libc-symbols.h:472:33: error: ‘__EI___strchrnul_c’ aliased to undefined symbol ‘__strchrnul_c’
  472 |   extern thread __typeof (name) __EI_##name \
      |                                 ^~~~~
./../include/libc-symbols.h:468:3: note: in expansion of macro ‘__hidden_ver2’
  468 |   __hidden_ver2 (, local, internal, name)
      |   ^~~~~~~~~~~~~
../sysdeps/s390/strchrnul-c.c:28:1: note: in expansion of macro ‘__hidden_ver1’
   28 | __hidden_ver1 (__strchrnul_c, __GI___strchrnul, __strchrnul_c);
      | ^~~~~~~~~~~~~
make[2]: *** [/home/dilfridge/Gentoo/misc/glibc-build/sysd-rules:603: /home/dilfridge/Gentoo/misc/glibc-build/string/strchrnul-c.os] Error 1
make[2]: *** Waiting for unfinished jobs....
make[2]: Leaving directory '/home/dilfridge/Gentoo/misc/glibc/string'
make[1]: *** [Makefile:484: string/subdir_lib] Error 2
make[1]: Leaving directory '/home/dilfridge/Gentoo/misc/glibc'
make: *** [Makefile:9: all] Error 2

The recent s390 string refactor assumed multiarch/ifunc support as default and organized the internal code without placing the ifunc selector/variants only on multiarch folder.  This is to allow the avoid ifunc and some string variants depending of the default compiler optimization (so if you target glibc to a z13, there is no need to provide old z900 variants and glibc internally can assume the z13 variants as default).

I am not very found of this s390-specific organization, it differs from other ports where there is no need to extra internal checks to see if you are building with multiarch support (USE_MULTIARCH). The following patch should fix s390 and s390x, but I think it would be better to make s390 follow current practice of putting ifunc support only at multiarch folders.

diff --git a/sysdeps/s390/memrchr-c.c b/sysdeps/s390/memrchr-c.c
index bdf3c7bbe0..3cb27b795d 100644
--- a/sysdeps/s390/memrchr-c.c
+++ b/sysdeps/s390/memrchr-c.c
@@ -25,7 +25,7 @@

 # include <string/memrchr.c>

-# if defined SHARED && IS_IN (libc)
+# if defined SHARED && IS_IN (libc) && defined USE_MULTIARCH
 __hidden_ver1 (__memrchr_c, __GI___memrchr, __memrchr_c);
 # endif
 #endif
diff --git a/sysdeps/s390/strchrnul-c.c b/sysdeps/s390/strchrnul-c.c
index f6f5bae311..cee0e1a89e 100644
--- a/sysdeps/s390/strchrnul-c.c
+++ b/sysdeps/s390/strchrnul-c.c
@@ -24,7 +24,7 @@
 # endif

 # include <string/strchrnul.c>
-# if defined SHARED && IS_IN (libc)
+# if defined SHARED && IS_IN (libc) && defined USE_MULTIARCH
 __hidden_ver1 (__strchrnul_c, __GI___strchrnul, __strchrnul_c);
 # endif
 #endif

STRCHRNUL is defined before including string/strchrnul.c if HAVE_STRCHRNUL_IFUNC is not zero. Thus the hidden symbol should also depend on HAVE_STRCHRNUL_IFUNC instead of USE_MULTIARCH.

I will take care of it

We have a few --disable-multi-arch variants in build-many-glibcs.py, I guess some should be added for s390 as well.

I've just posted this patch:
[PATCH] S390: Fix building with --disable-mutli-arch [BZ #31196]
https://sourceware.org/pipermail/libc-alpha/2024-January/154321.html

Sure, I can also add some --disable-mutli-arch tests in build-many-glibcs.py. I will post a separate patch.

Andreas has just committed my patch to be within glibc 2.39 release:
"S390: Fix building with --disable-mutli-arch [BZ #31196]"
https://sourceware.org/git/?p=glibc.git;a=commit;h=cc1b91eabd806057aa7e3058a84bf129ed36e157

I will cherry-pick it to glibc 2.38 branch. Once done, I close this bug.
@Andreas Huettel: Is gentoo following the release-branches or do you pick the commit up by yourself?

FYI: I've also posted this patch:
[PATCH] build-many-glibcs.py: Add s390 --disable-multi-arch / multi-arch configurations.
https://sourceware.org/pipermail/libc-alpha/2024-January/154342.html

(In reply to Stefan Liebler from comment #9)
> Andreas has just committed my patch to be within glibc 2.39 release:
> "S390: Fix building with --disable-mutli-arch [BZ #31196]"
> https://sourceware.org/git/?p=glibc.git;a=commit;
> h=cc1b91eabd806057aa7e3058a84bf129ed36e157
> 
> I will cherry-pick it to glibc 2.38 branch. Once done, I close this bug.
> @Andreas Huettel: Is gentoo following the release-branches or do you pick
> the commit up by yourself?
> 
> FYI: I've also posted this patch:
> [PATCH] build-many-glibcs.py: Add s390 --disable-multi-arch / multi-arch
> configurations.
> https://sourceware.org/pipermail/libc-alpha/2024-January/154342.html

Gentoo is following the release branches, I'll pick the branch up for the next revision. Thank you!

(And sorry, I knew somewhere but at the moment completely forgot about the typo...)

Also pushed it to release/2.38/master:
"S390: Fix building with --disable-mutli-arch [BZ #31196]"
https://sourceware.org/git/?p=glibc.git;a=commit;h=18876c9ff52c3d9aefe2c663b1a287589bebedc0