Disable read barrier thunks for heap poisoning

Logic to unpoison is not yet implemented, this causes SIGSEGVs in
various places.

Bug: 29516974
Bug: 30126666
Bug: 36141117

Test: ./test/testrunner/testrunner.py -j4 --optimizing --debuggable --ndebuggable --target --verbose
Change-Id: I8317a142d07af36090d5f05ce46100dfa07c17e7