This reworks `set_native_thread_name()` for macOS to not rely on mach thread ports returned server side (which were potentially colliding cross-process anyways) and re-implements what `pthread_setname_np()` would do on wines side, with additionally allowing setting threadnames cross-process.
On the kernel side `bsd_setthreadname()` is apparently "known to race with regards to the contents of the thread name", so that is only exposed for the thread calling it on itself.
Since the pthread struct is private, the location of the thread name is determined at runtime (although in practice, it is constant (80 bytes) from at least 10.15 up to 14.0).
This has an effect on both `pthread_getname_np()` and the `NSThread` implementation on top a native library would use.
Cross-thread naming is only not visible to external debuggers, which have acquired a mach port right and calling `thread_info()` with `THREAD_EXTENDED_INFO`
-- v2: ntdll: Reimplement set_native_thread_name() for macOS.