nptl: Move pthread_yield into libc, as a compatibility symbol

And deprecate it in <pthread.h>, redirecting it to sched_yield
for the time being.

The symbol was moved using scripts/move-symbol-to-libc.py.

No GLIBC_2.34 symbol version is added because of the compatibility
symbol status.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>