Per my reading of the OpenACC specification (and as supported by secondary
documentation, such as code examples, or presentations), it's valid to call
"acc_get_cuda_stream"/"acc_set_cuda_stream" also with "acc_async_sync",
"acc_async_noval" arguments, not just with the nonnegative values as currently
implemented.

	libgomp/
	PR libgomp/88370
	* libgomp.texi (acc_get_current_cuda_context, acc_get_cuda_stream)
	(acc_set_cuda_stream): Clarify.
	* oacc-cuda.c (acc_get_cuda_stream, acc_set_cuda_stream): Use
	"async_valid_p".
	* plugin/plugin-nvptx.c (nvptx_set_cuda_stream): Refuse "async ==
	acc_async_sync".
	* testsuite/libgomp.oacc-c-c++-common/acc_set_cuda_stream-1.c: New file.
	* testsuite/libgomp.oacc-c-c++-common/async_queue-1.c: Likewise.
	* testsuite/libgomp.oacc-c-c++-common/lib-84.c: Update.
	* testsuite/libgomp.oacc-c-c++-common/lib-85.c: Likewise.

From-SVN: r267147