cmake: do not pass $SIMD_COMPILE_FLAGS to rocksdb cmake

cmake: do not pass $SIMD_COMPILE_FLAGS to rocksdb cmake

which enables SSE42 globally in rocksdb. and we will end up with a
binary not portable on non-SSE42 enabled machines.

Fixes: http://tracker.ceph.com/issues/20529
Signed-off-by: Kefu Chai <kchai@redhat.com>
Conflicts:
	this change is not cherry-picked from master. because the
PR targeting master (https://github.com/ceph/ceph/pull/17388) is
still pending on review. and the cmake changes is different if
we want to use a recent commit of rocksdb, as it's doing differently
in cmake to address the portability issues.