RFR (sh/jdk8): Cleanup weak JNI refs when not doing reference processing

[Roman Kennke]

We may run into troubles in jdk8 when doing class-unloading but not 
reference-processing: in JDK8, ref-processing also handles JNI weak-refs 
(it doesn't really belong there, which is why it has been moved out in 
later releases). This means that when we have a cycle that doesn't do 
ref-processing, we may skip cleaning JNI refs and end up with dangling 
refs there. This manifests easily when running IntelliJ.

Fix is to clean JNI weak-refs explicitely when skipping ref-processing.

Fix:
http://cr.openjdk.java.net/~rkennke/shjdk8-cleanup-weakjni/webrev.00/

Testing: hotspot_gc_shenandoah
IntelliJ with default and traversal mode (failed before/passes now)

Good?

Thanks,
Roman