posix: pthread: implement pthread_atfork()

pthread_atfork() is required by the POSIX_THREADS_BASE
Option Group as detailed in Section E.1 of IEEE-1003.1-2017.

The POSIX_THREADS_BASE Option Group is required for PSE51,
PSE52, PSE53, and PSE54 conformance, and is otherwise
mandatory for any POSIX conforming system as per Section
A.2.1.3 of IEEE-1003-1.2017.

Since Zephyr does not yet support processes and (by extension)
fork(), this implementation includes a deviation and should be
categorized as producing undefined behaviour.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>