6820167: GCALotAtAllSafepoints + FullGCALot(ScavengeALot) options crash JVM

Short-circuit gc-a-lot attempts by non-JavaThreads; SkipGCALot c'tor to elide re-entrant gc-a-lot attempts.

Reviewed-by: apetrusenko, jcoomes, jmasa, kamg

hotspot/src/share/vm/runtime/vmThread.cpp

@@ -531,6 +531,7 @@ void VMThread::execute(VM_Operation* op) {
   Thread* t = Thread::current();
 
   if (!t->is_VM_thread()) {
+    SkipGCALot sgcalot(t);    // avoid re-entrant attempts to gc-a-lot
     // JavaThread or WatcherThread
     t->check_for_valid_safepoint_state(true);