RFR: Avoid Full STW GC on System.gc()

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/concthread-explicitgc/webrev.01/

Does what is says: triggers concurrent cycle on System.gc() now. Tests DisableExplicitGC and
ExplicitGCInvokesConcurrent options too.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey