x/sys/unix: add darwin efficiency core scheduling integer constants

Proposal Details

Darwin has the ability to force scheduling to the efficiency cores on Apple Silicon through the setpriority syscall, but the integer constants currently aren't exposed.

I think changing the regex here: cs.opensource.google / github from PRIO_(PROCESS|PGRP|USER) to PRIO_(PROCESS|PGRP|USER|DARWIN_THREAD|DARWIN_PROCESS|DARWIN_BG|DARWIN_NONUI) or PRIO_(PROCESS|PGRP|USER|DARWIN_(THREAD|PROCESS|BG|NONUI)) would result in being able to force execution onto an efficiency core.

As an additional reference, here's the CPython PR: https://github.com/python/cpython/issues/103545

I'd love to actually land this change in Go myself, but frankly don't know where to begin.