[og9] Improve async serialize implementation for AMD GCN libgomp plugin

[og9] Improve async serialize implementation for AMD GCN libgomp plugin

	libgomp/
	* plugin/plugin-gcn.c (struct placeholder, struct asyncwait_info,
	enum entry_type): New.
	(queue_entry): Use entry_type enum for tag.  Add asyncwait and
	placeholder event type fields.
	(wait_for_queue_nonfull): New function.
	(queue_push_launch): Use above function instead of raising a fatal
	error on queue-full condition.  Use KERNEL_LAUNCH instead of hardwired
	0.
	(queue_push_callback): Use wait_for_queue_nonfull instead of open-coded
	wait sequence.  Use CALLBACK instead of hardwired 1.
	(queue_push_asyncwait, queue_push_placeholder): New.
	(execute_queue_entry): Implement ASYNC_WAIT and ASYNC_PLACEHOLDER event
	types.
	(GOMP_OFFLOAD_openacc_async_serialize): Use queue_push_placeholder and
	queue_push_asyncwait instead of host-synchronized wait_queue calls.
	* testsuite/libgomp.oacc-c-c++-common/data-2-lib.c (main): Add missing
	asynchronous waits.
	* testsuite/libgomp.oacc-c-c++-common/data-2.c (main): Likewise.

(cherry picked from openacc-gcc-9-branch commit
714b46df96da40f15b38757bc8900451892f32be)