RFR: 8202722: Move marksweep_init into GC code

[Stefan Karlsson]

Hi all,

Please review this patch to move marksweep_init into GC code.

http://cr.openjdk.java.net/~stefank/8202722/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8202722

marksweep_init is guarded by INCLUDE_SERIALGC in init.cpp. I propose 
that we move this code to MarkSweep and only call it when actually 
setting up GCs using MarkSweep.

Thanks,
StefanK