C++11 requires <atomic>. This lets us remove the header detection
(LEVELDB_ATOMIC_PRESENT) and simplify port/atomic_pointer.h.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=189919098

Fixes https://github.com/google/leveldb/issues/466

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=189425354