RFR: 8202813: Move vm_weak processing from SystemDictionary::do_unloading to WeakProcessor

[Kim Barrett]

Please review this change to move the clearing of dead entries in
vm_weak_oop_storage from SystemDictionary::do_unloading to preceeding
calls to WeakProcessor::weak_oops_do.

This involves conditionalizing WeakProcessor invocations, to allow
different calls to process different subsets of the native weak
references.  For example, vm_weak_oop_storage is treated as strong
rather than weak when ClassUnloading is disabled.

CR:
https://bugs.openjdk.java.net/browse/JDK-8202813

Webrev:
http://cr.openjdk.java.net/~kbarrett/8202813/open.00/

Testing:
Mach5 jdk-tier{1-2}, hs-tier{1-5}
Locally runthese30m.