Summary:
If possible, use -march or -mcpu to get enable all features available on the local CPU or architecture. Only if this is impossible, we will manually set -msse4.2. It should be safe as there'll be a warning printed if `USE_SSE` is set and the provided flags are insufficient to support SSE4.2.
Closes https://github.com/facebook/rocksdb/pull/3156
Differential Revision: D6304703
Pulled By: ajkr
fbshipit-source-id: 030a53491263300cae7fafb429114d87acc828ef