use __atomics in SkBarriers_tsan.h

I am having a hard time getting TSAN's own atomic hooks to work on our bots,
but __atomic_foo work fine and are understood just fine by TSAN.

BUG=skia:
R=bungeman@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/320063003
1 file changed