An OpenMP "nowait" clause on a target construct currently leads to
a call to GOMP_OFFLOAD_async_run in the plugin that is used for
offloading at execution time. The nvptx plugin contains only a stub
of this function that always produces a fatal error if called.

This commit changes the "nowait" implementation to ignore the clause
if the executing device's plugin does not implement GOMP_OFFLOAD_async_run.
The stub in the nvptx plugin is removed which effectively means that
programs containing "nowait" can now be executed with nvptx offloading
as if the clause had not been used.
This behavior is consistent with the OpenMP specification which says that
"[...] execution of the target task *may* be deferred" (emphasis added),
cf. OpenMP 5.0, page 172.

libgomp/

	* plugin/plugin-nvptx.c: Remove GOMP_OFFLOAD_async_run stub.
	* target.c (gomp_load_plugin_for_device): Make "async_run" loading
	optional.
	(gomp_target_task_fn): Assert "devicep->async_run_func".
	(clear_unsupported_flags): New function to remove unsupported flags
	(right now only GOMP_TARGET_FLAG_NOWAIT) that can be be ignored.
	(GOMP_target_ext): Apply clear_unsupported_flags to flags.
	* testsuite/libgomp.c/target-33.c:
	Remove xfail for offload_target_nvptx.
	* testsuite/libgomp.c/target-34.c: Likewise.