posix: pthread: rework pthreads to use zephyr api

Previously, pthreads suffered from some race conditions.
This was almost inevitable given that it was maintained in
parallel to Zephyr's threading and synchronization API.

The unfortunate side-effect of with that is that it did not
receive the reliability and other improvements that
`k_thread`s did.

Here, we perform a significant update of pthread code so
that it depends directly on public Zephyr API. With that,
we reuse as many concepts as possible and pthreads benefits for
free from any improvement made to Zephyr's threading and
synchronization APIs.

Included with this change, we
* implement state with `ready_q`, `run_q`, and `done_q`
* use `pthread_barrier_wait()` to sync `pthread_create()`
* synchronize internal state with a spinlock

These pthreads are considerably more reliable than
before.

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