posix: rwlock: implement pthread_rwlockattr_setpshared

Implement pthread_rwlockattr_setpshared() and
pthread_rwlockattr_getpshared().

Both functions are required by the _POSIX_READER_WRITER_LOCKS
Option as detailed in Section E.1 of IEEE-1003.1-2017.

The _POSIX_READER_WRITER_LOCKS Option 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.

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