Special case JIT update for native methods.

The update might apply to ArtMethods that are going to be
unloaded, so ensure we don't do read barriers there.

Test: while ./art/test/run-test --host  --no-dex2oat --jit --host --no-prebuild \
        --compact-dex-level none --dex2oat-jobs 4 --no-relocate --runtime-option -Xcheck:jni \
        --build-with-javac-dx 674-hiddenapi ; do true; done

Change-Id: I95ec6107c65da25f4b98f7fb77647b3ab382a93f