Loom: Basic continuation entry and yield support

[tajila]

Depends on #15172 and #15176

Create a J9VMContinuation

• allocate J9VMContinuation, perhaps out of pool
  • optionally add deadContinuationThreadList and use recycled ones 
• allocate java stack for J9VMContinuation
  • optionally allocate from pool as well
• add to continuation continuationlist
• assign J9VMContinuation to continuationObject->vmref
• register j9vmcontinuation with GC
• initialize J9VMContinuation interpreter fields

Enter operation

• set bit on j9vmthread->privateFlags to indicate running VirtualThread
• build call-in frame (if not started)
• swap carrierthread interpreter state with continuation
• set currentContinuation on carrier thread to new Continuation
• call into Continuation entry, if starting new
  • resume state otherwise
• once complete set continuation to finished
• unset vmref in Continuation instance and free/recycle j9VMcontinuation
• unset bit on j9vmthread->privateFlags

Yield operation

• check if pinned, posssibly assert if true (JCL should not attempt unmount if pinned)
• swap continuation interpreter state with saved carrier thread state
• unset currentContinuation in j9vmthread
• unset bit on j9vmthread->privateFlags
• resume carrier thread

class Continuation {
        //should be cINL
	private native boolean enter();
	
        //should be cINL
	private static native boolean yield();

	private native boolean createContinuationImpl();
}