RFR: 8183542: Factor out serial GC specific code from GenCollectedHeap into its own subclass

[Roman Kennke]

This gives the serial GC its own (Gen)CollectedHeap subclass. There is 
not much serial specific code in GCH, but it seems odd to report it as 
'serial' while its subclasses override it as CMS or Parallel.

Tested successfully with hotspot_gc.

http://cr.openjdk.java.net/~rkennke/8183542/webrev.00/ 
<http://cr.openjdk.java.net/%7Erkennke/8183542/webrev.00/>

Opinions? Ok to go in?

Thanks, Roman