ARM/AOT: Allow 16-bit LDR for Baker read barrier loads.

Test: m test-art-target-gtest
Test: testrunner.py --target on Nexus 6P.
Test: testrunner.py --target on Nexus 6P with heap poisoning enabled.
Test: Repeat the above tests with ART_USE_OLD_ARM_BACKEND=true.
Bug: 29516974
Bug: 30126666
Bug: 36141117
Change-Id: I458f2ec5fe9abead4db06c7595d992945096fb68
13 files changed