RFR: 8235757 : Rename SupportedGC to IncludedGC

[Stefan Karlsson]

Hi all,

Please review this trivial patch to rename SupportedGC (and associated 
macros and functions) to IncludedGC.

https://cr.openjdk.java.net/~stefank/8235757/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8235757

I want to do this name change to disambiguate an upcoming addition to 
GCConfig::is_gc_supported, where built-in GCs doesn't mean that the GC 
is supported on the current platform.

I'm using the name IncludedGC because it reflects our usage of 
INCLUDE_*GC macros and defines.

Thanks,
StefanK