RFR: 8013934: Garbage collection event for CMS has wrong cause for System.gc()

[Erik Helin]

Hi all,

this change fixes an issue with the cause field in the 
vm/gc/collector/garbage_collection trace event for CMS.

When executing a GC via a call to System.gc, then cause was not 
"System.gc" for CMS in the trace event as expected (it was "CMS 
concurrent mark").

The problem is that the VM operation VM_GenCollectFullConcurrent gets 
the cause as parameter but does not pass it on to the CMSCollector. This 
change adds the parameter "cause" to CMSCollector::request_full_gc and 
the CMSCollector then saves the parameter as CMSCollector::_full_gc_cause.

Webrev:
http://cr.openjdk.java.net/~ehelin/8013934/webrev.00/

Thanks,
Erik