Re-enable ART's region space memory protection on host.

Memory protection of unused regions was disabled on host because of
spurious errors due to old kernel issues on some machines. These
problems have presubmably been fixed since, so let's re-enable that
mechanism.

Test: art/test.py --host
Bug: 63131961
Bug: 74064045
Change-Id: Ieb03540fd205401a2b031e0d4c90444b5b2c8aed