Change one read barrier bit to mark bit

Change one read barrier bit to mark bit

Optimization to help slow path performance. When the GC marks an
object through the read barrier slow path. The GC sets the mark bit
in the lock word of that reference. This bit is checked from the
assembly entrypoint the common case is that it is set. If the bit is
set, the read barrier knows the object is already marked and there is
no work to do.

To prevent dirty pages in zygote and image, the bit is set by the
image writer and zygote space creation.

EAAC score (lower is better):
N9: 777 -> 700 (average 31 of runs)
N6P (960000 mhz): 1737.48 -> 1442.31 (average of 25 runs)

Bug: 30162165
Bug: 12687968

Test: N9, N6P booting, test-art-host, test-art-target all with CC

Change-Id: Iae0cacfae221e33151d3c0ab65338d1c822ab63d