[PATCH v6] posix: Fix system blocks SIGCHLD erroneously [BZ #30163]

[Joseph Myers]

This breaks the testsuite build for Hurd (tst-system needs to link with 
$(shared-thread-library), I think).

/scratch/jmyers/glibc-bot/install/compilers/i686-gnu/lib/gcc/i686-glibc-gnu/12.2.1/../../../../i686-glibc-gnu/bin/ld: /scratch/jmyers/glibc-bot/build/glibcs/i686-gnu/glibc/support/libsupport_nonshared.a(xpthread_create.oS): in function `xpthread_create':
/scratch/jmyers/glibc-bot/src/glibc/support/xpthread_create.c:26: undefined reference to `pthread_create'
/scratch/jmyers/glibc-bot/install/compilers/i686-gnu/lib/gcc/i686-glibc-gnu/12.2.1/../../../../i686-glibc-gnu/bin/ld: /scratch/jmyers/glibc-bot/build/glibcs/i686-gnu/glibc/support/libsupport_nonshared.a(xpthread_join.oS): in function `xpthread_join':
/scratch/jmyers/glibc-bot/src/glibc/support/xpthread_join.c:25: undefined reference to `pthread_join'
collect2: error: ld returned 1 exit status
make[3]: *** [../Rules:238: /scratch/jmyers/glibc-bot/build/glibcs/i686-gnu/glibc/stdlib/tst-system] Error 1

https://sourceware.org/pipermail/libc-testresults/2023q1/010979.html

-- 
Joseph S. Myers
joseph@codesourcery.com