RFR: 8200759: Move GC entries in vmStructs.cpp to GC specific files

[Stefan Karlsson]

Hi all,

Please review this patch to move GC specific entries in vmStructs.cpp 
out to GC specific files.

http://cr.openjdk.java.net/~stefank/8200759/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8200759

This patch is done in preparation for:
  https://bugs.openjdk.java.net/browse/JDK-8200729 - Conditional 
compilation of GCs

This patch also has the nice feature that it removes multiple usages of 
the GC specific defines in vmStructs.cpp. When new GCs are added only 
three defines need to be added to vmStructs_gc.hpp.

Tested locally with test/hotspot/jtreg/serviceability/sa, but will run 
this through mach5 to find potantial missing includes / forward 
declarations.

Thanks,
StefanK