RFR: 8201168: Move GC command line constraint functions to GC specific files

[Stefan Karlsson]

Hi all,

Please review this patch to move the GC flag constraint functions into 
GC specific files.

  http://cr.openjdk.java.net/~stefank/8201168/webrev.01
  https://bugs.openjdk.java.net/browse/JDK-8201168

This is one step towards:

  https://bugs.openjdk.java.net/browse/JDK-8200729 - Conditional 
compilation of GCs

I know that this conflicts with Gerards rename of the flags to JVMFlags, 
but I don't want to wait for that to get pushed before publishing this 
review request.

Thanks,
StefanK