ART: Emit runtime read barrier checks only in slow-debug

Move runtime read barrier check emission to slow-debug to have
better control over when this instrumentation is done.

Bug: 35644369
Bug: 68025088
Test: m test-art-host
Test: manual inspection of core image code
Change-Id: I69b3b6f243c30813a741fe9e0a460f1543c6bc5d