posix: pthread: consider PTHREAD_EXITED state in pthread_create

If a thread is joined using `pthread_join()`, then the
internal state would be set to `PTHREAD_EXITED`.

Previously, `pthread_create()` would only consider pthreads
with internal state `PTHREAD_TERMINATED` as candidates for new
threads. However, that causes a descriptor leak.

We should be able to reuse a single thread an infinite number
of times.

Here, we also consider threads with internal state
`PTHREAD_EXITED` as candiates in `pthread_create()`.

Fixes #47609

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