OpenJDK 6 and 7 issues with CMSClassUnloadingEnabled being set without UseConcMarkSweepGC

OpenJDK 6 and 7 by default have UseConcMarkSweepGC disabled. Thus, setting just CMSClassUnloadingEnabled will not work as intended. This can be verified by examining the OpenJDK 6 and 7 source. /hotspot/src/share/vm/runtime/globals.hpp declares it as false by default and the only GC where it is used is /hotspot/src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.cpp.

Note: It is possible for the CMS GC to be automatically selected, as defined in arguments.cpp. However, explicitly stating it would assure its use.