Add mac/ios pthread_from_mach_thread_np

Allows us to get a pthread id from a match thread id.
From pthread.h [1]:

```
__API_AVAILABLE(macos(10.5), ios(2.0))
_Nullable pthread_t pthread_from_mach_thread_np(mach_port_t);
```

[1] - https://opensource.apple.com/source/libpthread/libpthread-416.40.3/pthread/pthread.h.auto.html