try un-teaching TSAN about Mac semaphores
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 <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Commit-Queue: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>
Reviewed-by: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>

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