RFR (M) 8202813: Move vm_weak processing from SystemDictionary to WeakProcessor

[Kim Barrett]

> On May 25, 2018, at 11:32 AM, coleen.phillimore at oracle.com wrote:
> 
> Summary: SystemDictionary has all strong roots.  The weak oop_storage is processed by the WeakProcessor so it can be scanned and cleared concurrently and/or by parallel threads.
> 
> Please see bug for explanation.
> 
> open webrev at http://cr.openjdk.java.net/~coleenp/8202813.01/webrev
> bug link https://bugs.openjdk.java.net/browse/JDK-8202813
> 
> There might be more GC code that can be cleaned out since G1RootClosures->weak_oops() seems unused now, but I'd like to defer that to a smaller cleanup.  Also ClassLoaderData is still processed inconsistently by the GCs, which would be nice to clean up.
> 
> Tested with mach5 hs-tier1-5 with no failures.  Per's gc-test-suite, dev-submit for performance testing, and runThese with all collectors, with and without -XX:-ClassUnloading.  Also tested with class unloading gc tests (which are also in hs-tier number tests).
> 
> Thanks,
> Coleen

Looks good.