The following error is generated by the assembler when attempting to build glibc
2.7 on x86_64.  Perhaps a compiler issue, but I got no complaint from configure. 

gcc ../sysdeps/x86_64/mempcpy.S -c  -I../include
-I/home/acbell/glibc-build/string -I/home/acbell/glibc-build
-I../sysdeps/x86_64/elf -I../nptl/sysdeps/unix/sysv/linux/x86_64
-I../sysdeps/unix/sysv/linux/x86_64 -I../sysdeps/unix/sysv/linux/wordsize-64
-I../nptl/sysdeps/unix/sysv/linux -I../nptl/sysdeps/pthread -I../sysdeps/pthread
-I../sysdeps/unix/sysv/linux -I../sysdeps/gnu -I../sysdeps/unix/common
-I../sysdeps/unix/mman -I../sysdeps/unix/inet -I../nptl/sysdeps/unix/sysv
-I../sysdeps/unix/sysv -I../sysdeps/unix/x86_64 -I../nptl/sysdeps/unix
-I../sysdeps/unix -I../sysdeps/posix -I../sysdeps/x86_64/fpu
-I../nptl/sysdeps/x86_64 -I../sysdeps/x86_64 -I../sysdeps/wordsize-64
-I../sysdeps/ieee754/ldbl-96 -I../sysdeps/ieee754/dbl-64
-I../sysdeps/ieee754/flt-32 -I../sysdeps/ieee754 -I../sysdeps/generic/elf
-I../sysdeps/generic -I../nptl  -I.. -I../libio -I. -I
/lib/modules/2.6.9-22.0.1.ELsmp/build/include -D_LIBC_REENTRANT -include
../include/libc-symbols.h       -DASSEMBLER   -Wa,--noexecstack  -o
/home/acbell/glibc-build/string/mempcpy.o -MD -MP -MF
/home/acbell/glibc-build/string/mempcpy.o.dt -MT
/home/acbell/glibc-build/string/mempcpy.o
../sysdeps/x86_64/memcpy.S: Assembler messages:
../sysdeps/x86_64/memcpy.S:513: Error: suffix or operands invalid for `movnti'
../sysdeps/x86_64/memcpy.S:514: Error: suffix or operands invalid for `movnti'
../sysdeps/x86_64/memcpy.S:515: Error: suffix or operands invalid for `movnti'
../sysdeps/x86_64/memcpy.S:516: Error: suffix or operands invalid for `movnti'
../sysdeps/x86_64/memcpy.S:517: Error: suffix or operands invalid for `movnti'
../sysdeps/x86_64/memcpy.S:518: Error: suffix or operands invalid for `movnti'
../sysdeps/x86_64/memcpy.S:519: Error: suffix or operands invalid for `movnti'
../sysdeps/x86_64/memcpy.S:520: Error: suffix or operands invalid for `movnti'
../sysdeps/x86_64/memcpy.S:531: Error: suffix or operands invalid for `movnti'
../sysdeps/x86_64/memcpy.S:532: Error: suffix or operands invalid for `movnti'
../sysdeps/x86_64/memcpy.S:533: Error: suffix or operands invalid for `movnti'
../sysdeps/x86_64/memcpy.S:534: Error: suffix or operands invalid for `movnti'
../sysdeps/x86_64/memcpy.S:535: Error: suffix or operands invalid for `movnti'
../sysdeps/x86_64/memcpy.S:536: Error: suffix or operands invalid for `movnti'
../sysdeps/x86_64/memcpy.S:537: Error: suffix or operands invalid for `movnti'
../sysdeps/x86_64/memcpy.S:538: Error: suffix or operands invalid for `movnti'
gmake[2]: *** [/home/acbell/glibc-build/string/mempcpy.o] Error 1

From config.log:

$ ../glibc-2.7/configure --prefix=/home/acbell/glibc

hostname = galaga.cssm.iastate.edu
uname -m = x86_64
uname -r = 2.6.9-22.0.1.ELsmp
uname -s = Linux
uname -v = #1 SMP Tue Oct 18 18:39:02 EDT 2005

$ gcc -v
Reading specs from /usr/lib/gcc/x86_64-redhat-linux/3.4.4/specs
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --enable-shared --enable-threads=posix
--disable-checking --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-java-awt=gtk --host=x86_64-redhat-linux
Thread model: posix
gcc version 3.4.4 20050721 (Red Hat 3.4.4-2)