Avoid CAS for marking region space bitmap for baker

Avoid CAS for marking region space bitmap for baker

Only have the GC thread mark it. This occurs when popping from the
mark stack. The race where an object may be pushed to the mark
stack twice is handled by not scanning if it is already marked.

Also avoid checking is_active when marking from the GC.

EAAC: 1263 -> 1253 (average of 30 runs)
GC time: 7.21s -> 6.83s (average of 18 runs)

Timings on 960 mhz N6P.

Bug: 12687968

Change-Id: I47e98c3e258829d2ba0babd803a219c82a36168c
Test: test-art-host, debug N6P booting with baker CC.