RFR (S): jweak handling misses barriers

[Aleksey Shipilev]

Hi,

This is the post-merge trouble. There are new places where G1 added barriers for
handling JNI references. We should follow suit:
  http://cr.openjdk.java.net/~shade/shenandoah/jweak-barriers/webrev.01/

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey