Use the "GC is marking" information in compiler read barriers (ARM, ARM64).

Use the "GC is marking" information in compiler read barriers (ARM, ARM64).

In compiler-generated code, when deciding whether to mark
a heap reference or not in a read barrier, check whether
the GC is currently marking, instead of checking the gray
bit in the reference's holder's lock word.

This change is only for ARM and ARM64, as it does not
benefit x86 nor x86-64.

Test: Run ART tests in Baker read barrier configuration.
Test: Boot a device in Baker read barrier configuration.
Bug: 29516974
Change-Id: Ia5d90286bb9f753f3bbcb3a6254eb166523a2ff5