runtime: move semaphore ops from system calls to libc calls on Darwin

This CL removes the last of the direct system calls in the runtime package.
This is the last CL for 1.11.

Use libcCall instead of asmcgocall in a few places I accidentally used
the wrong one.

For 1.12, we need to think about whether/how the syscall package
should be moved over to libc.

Update #17490

Change-Id: I4f0bd9cd6023f662f2e29588266fdfae5233898f
Reviewed-on: https://go-review.googlesource.com/118736


Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>