Fix gtest failures due to userfaultfd GC

Mostly involved replacing compile-test check for read-barrier with
runtime one.

Bug: 160737021
Test: art/test/testrunner/run_build_test_target.py -j <core-count> art-test
Change-Id: I685e4b717d314a19f7714ece5c8b84f23154cc66
4 files changed