ARM64: Use link-time generated thunks for Baker CC read barrier.

Remaining work for follow-up CLs:
  - array loads,
  - volatile field loads,
  - use implicit null check in field thunk.

Test: Added tests to relative_patcher_arm64
Test: New run-test 160-read-barrier-stress
Test: m test-art-target-gtest on Nexus 6P.
Test: testrunner.py --target on Nexus 6P.
Bug: 29516974
Bug: 30126666
Bug: 36141117
Change-Id: Id68ff171c55a3f1bf1ac1b657f480531aa7b3710