Revert "runtime: use dispatch semaphores on Darwin"

Revert "runtime: use dispatch semaphores on Darwin"

This reverts https://golang.org/cl/182258.

The new code caused unpredictable crashes that are not understood.  The old code was occasionally flaky but still better than this approach.

Fixes #32655
Updates #31264

Change-Id: I2e9d27d6052e84bf75106d8b844549ba4f571695
Reviewed-on: https://go-review.googlesource.com/c/go/+/182880
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>