Don't do a read barrier in JIT GC code.

The ArtMethod the JIT GC iterates over might be in the
process of being unloaded.

bug: 74369794
Test: test.py --jit
Test: test-art-host-run-test-debug-no-prebuild-jit-no-relocate-ntrace-cms-checkjni-picimage-npictest-ndebuggable-no-jvmti-cdex-fast-674-hiddenapi64
Change-Id: Ibf8acb0df26f90c479420ef9d0c1b1bc1fcd3b0d
2 files changed