6984287: Regularize how GC parallel workers are specified

Associate number of GC workers with the workgang as opposed to the task.

Reviewed-by: johnc, ysr

hotspot/src/share/vm/gc_implementation/parNew/parNewGeneration.cpp

@@ -1533,3 +1533,7 @@ void ParNewGeneration::ref_processor_init()
 const char* ParNewGeneration::name() const {
   return "par new generation";
 }
+
+bool ParNewGeneration::in_use() {
+  return UseParNewGC && ParallelGCThreads > 0;
+}