enable O2 optimization for lz4
enable O2 optimization for lz4

Summary: Closes https://github.com/facebook/rocksdb/pull/2164

Differential Revision: D4897389

Pulled By: yiwu-arbug

fbshipit-source-id: fac15374ae7fef1ece70fd2b9018f2451f3c2f7c

vmx/rocksdb,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,vmx/rocksdb,bbiao/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,Andymic/rocksdb,bbiao/rocksdb,Andymic/rocksdb,vmx/rocksdb,Andymic/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,vmx/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,Andymic/rocksdb,vmx/rocksdb,vmx/rocksdb,Andymic/rocksdb,bbiao/rocksdb,vmx/rocksdb