os/bluestore: enable SSE-assisted CRC32 calculations in RocksDB.

os/bluestore: enable SSE-assisted CRC32 calculations in RocksDB.

By default RocksDB extensively employs CRC32. It has two paths
for the checksum calculation:
 * rocksdb::crc32c::Slow_CRC32,
 * rocksdb::crc32c::Fast_CRC32.

The fast path depends on a run-time discovery of CPU capabilities
AND a compile-time define __SSE4_2__. Although my systems really
offer SSE4.2 support, the macro was undefined resulting in poor
RocksDB performance visible especially during WAL transactions.

The patch (awkwardly) adds the -msse4.2 to CXXFLAGS for RocksDB.

Signed-off-by: Radoslaw Zarzynski <rzarzynski@mirantis.com>