posix: pthread: fixes for coverity 321140 and 321092

The `pthread_once_lock` `k_mutex` is statically initialized and
only visible within file scope. Coverity identified it as unsafe
because the return values of `pthread_mutex_lock()` and
`pthread_mutex_unlock()` were unchecked. However, if those
functions were to fail here, it would be indicative that
something far worse has happened.

In any case, we add assertions that these functions
succeed rather than silently ignoring with `(void)`, which
ensures that we have coverage when assertions are enabled,
in test, while removing unneeded code with assertions disable,
in production.

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