updated warning about using OSMemoryBarrier for new versions of MacOS per https://github.com/google/leveldb/pull/422/files#diff-b20d9097440605779c4984e3a1346085R61
updated warning about using OSMemoryBarrier for new versions of MacOS per https://github.com/google/leveldb/pull/422/files#diff-b20d9097440605779c4984e3a1346085R61

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