unix: Implement -X realtime command-line option on macOS.

This adds a new command line option to the unix port `-X realtime` to
enable realtime priority on threads.  This enables high precision timers
for applications that need more accurate timers.

Related docs:
https://developer.apple.com/library/archive/technotes/tn2169/_index.html

Fixes issue #8621.

Signed-off-by: David Lechner <david@pybricks.com>

docs/unix/quickref.rst

@@ -73,6 +73,8 @@ General options:
     - ``-X heapsize=<n>[w][K|M]`` sets the heap size for the garbage collector.
       The suffix ``w`` means words instead of bytes. ``K`` means x1024 and ``M``
       means x1024x1024.
+    - ``-X realtime`` sets thread priority to realtime. This can be used to
+      improve timer precision. Only available on macOS.