jni: Add read barrier fast path to jni compiler

Static method dispatch via JNI requires a read barrier
for the ArtMethod::GetDeclaringClass() load before adding it to the
JNI StackHandleScope.

We used to call ReadBarrierJni unconditionally but add a branch
to skip calling it if the GC is not currently in the marking phase.

Test: ART_USE_READ_BARRIER=true make test-art-host test-art-target
Bug: 30437917
Change-Id: I4f505ebde17c0a67209c7bb51b3f39e37a06373a