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

(cherry picked from commit 34088e16c93f482c251e55351022762760500b63)

Change-Id: I1740fd178e7ad100b8e118452a4eb134b937992d
2 files changed