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>