try un-teaching TSAN about Mac semaphores

Now using dispatch_semaphore_t, maybe it already knows?

Change-Id: I65499ceaee195293541a37704ae424bbee7c8740
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/248456
Auto-Submit: Mike Klein <mtklein@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
1 file changed