SkAtomic: always use std::atomic

We were doing it on Windows, now do it everywhere.
This just changes the backend.  We could think about another step to actually
replacing all our sk_atomic_... with std atomic stuff.

CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-TSAN-Trybot
TBR=reed@google.com
Only deleting from include/...

Review URL: https://codereview.chromium.org/1441773002
7 files changed