posix: pthread: do not assert in pthread_exit() on k_thread

If `pthread_exit()` is called from a `k_thread`, then we would
previously trigger an assertion. The problem with that, is that
is POSIX is acting as a compatibility layer.

Given that it is a reasonable expectation to have the calling
thread exit or abort when calling `pthread_exit()`, lets do just
that.

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