htl: Move pthread_atfork to libc_nonshared.a

This follows bd60ce86 ('nptl: Move pthread_atfork to libc_nonshared.a')
with the same rationale: there is no non-libpthread equivalent to be used
for making linking against libpthread optional.

libpthread_nonshared.a is unused after this, so remove it from the
build.

There is no ABI impact because pthread_atfork was implemented using
__register_atfork in libc even before this change.

pthread_atfork has to be a weak alias because pthread_* names are not
reserved in libc.