Teach TSAN directly about semaphore_t.

Instead of teaching TSAN than SkMutex is a lock to get around it not
understanding Mach semaphore_t routines, teach it that there is a
happens-before relationship between semaphore_signal() and
semaphore_wait().

This reverts commit e395bf2d189e22822ddf2b46541c510d6d8fbcc0.
New changes are entirely restricted to SkSemaphore.cpp.

Change-Id: I27f647b93c48e81e8327db849881d669c4cd3d04
Reviewed-on: https://skia-review.googlesource.com/49180
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
4 files changed