cmake: build portable rocksdb

cmake: build portable rocksdb

* so rocksdb is not built with -march=native, because
  - this is not portable
  - -march=native is not supported by older version of GCC on aarch64.
* and drop the sse42 specific settings for rocksdb. since rocksdb uses
  "target" attribute to build sse42 optimized crc32, as long as the
  compiler on building host is able to emit sse42 instructions.
  see https://github.com/facebook/rocksdb/pull/2807

Fixes: http://tracker.ceph.com/issues/20529
Signed-off-by: Kefu Chai <kchai@redhat.com>