posix: pthread: implement pthread_cleanup_push() / pop()

pthread_cleanup_push() and pthread_cleanup_pop() are 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.

In this change, we require the addition of a dedicated
pthread_key_t that will not be available for applilcation usage.

Rather than including that as part of
CONFIG_MAX_PTHREAD_KEY_COUNT, we increase the storage by 1 in
order to be least invasive from the application perspective.

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