Refactor Baker CC read barrier link-time thunk generation.

Do not keep any knowledge of Baker thunk type in the
ArmBaseRelativePatcher, keep it strictly within the
derived classes.

Test: Nexus 6P boots.
Test: testrunner.py --target on Nexus 6P
Bug: 29516974
Bug: 30126666
Bug: 36141117
Change-Id: I7d9efcd2df1439bb51985467c5e3e53ddeb5a293