RFR: 8235759: Extend GCConfig::is_gc_supported to mean supported not only included in build

[Stefan Karlsson]

Hi all,

Please review this trivial patch to allow GC to say that they are 
unsupported even though they exists in the binary.

https://cr.openjdk.java.net/~stefank/8235759/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8235759

There's no change in behavior, but allows ZGC to specialize 
is_gc_supported in an up-coming patch.

This patch applies on-top of:
  8235757: Rename SupportedGC to IncludedGC

Thanks,
StefanK