RFR: 8201136: Move GC flags from globals.hpp to GC specific files

[Stefan Karlsson]

Hi all,

Please review this patch to move GC flags out of globals.hpp and into GC 
specific files.

  http://cr.openjdk.java.net/~stefank/8201136/webrev.01
  https://bugs.openjdk.java.net/browse/JDK-8201136

This is one step towards:
  https://bugs.openjdk.java.net/browse/JDK-8200729 - Conditional 
compilation of GCs

With this patch we only have to update gc_globals.hpp when adding new GCs.

Thanks,
StefanK