folly: avoid using atomic operations android can't handle
folly: avoid using atomic operations android can't handle

Summary:On Android, clang 3.8 + libstdc++ doesn't support the full
range of atomic operations.  This diff fixes up the ones we
ran into when building fb4a.

Reviewed By: mzlee

Differential Revision: D3092352

fb-gh-sync-id: 7fea8513e23425fc37050ad14d82aabaceb00352
shipit-source-id: 7fea8513e23425fc37050ad14d82aabaceb00352

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