Changes Darwin semaphore support from using pthread mutexes and
condition variables to using dispatch semaphores. Signaling a dispatch
semaphore is async-signal-safe.

Fixes #31264

Change-Id: If0ce47623501db13e3804b14ace5f4d8eaef461e
Reviewed-on: https://go-review.googlesource.com/c/go/+/182258Reviewed-by: Elias Naur <mail@eliasnaur.com>
Reviewed-by: Keith Randall <khr@golang.org>