posix: use uint32_t instead of void* for pthread_t

Several other widely-used pthread implementations
abstract `pthread_t` as `uint32_t`. The benefit
there is that we avoid passing around a pointer to
an internal structure (implementation detail).

Additionally, this removes the alias from `k_tid_t`
to `pthread_t` inside of `struct pthread_mutex`.

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