Need writeBarrier between resetting the state and freeContinuation

For weak memory ordering system such as PowerPC, we need to writeBarrier
between resetting the state of continuation(set the state to unmounted)
and freeing continuation/postUnmountContinuation during continuation
unmounting to avoid potential assert/issue caused by inconsist state
(in case the instructions are reordered).

Signed-off-by: hulin <linhu@ca.ibm.com>

Author: LinHu2016

runtime/vm/ContinuationHelpers.cpp

@@ -306,6 +306,7 @@ yieldContinuation(J9VMThread *currentThread, BOOLEAN isFinished)
 	 */
 	Assert_VM_true(VM_ContinuationHelpers::isMountedWithCarrierThread(*continuationStatePtr, currentThread));
 	VM_ContinuationHelpers::resetCarrierID(continuationStatePtr);
+	VM_AtomicSupport::writeBarrier();
 	/* Logically postUnmountContinuation(), which add the related continuation Object to the rememberedSet or dirty the Card for concurrent marking for future scanning, should be called
 	 * before resetCarrierID(), but the scan might happened before resetCarrierID() if concurrent card clean happens, then the related compensating scan might be
 	 * missed due to the continuation still is stated as mounted(we don't scan any mounted continuation, it should be scanned during root scanning via J9VMThread->currentContinuation).