Wait at end of OpenACC asynchronous kernels regions

Wait at end of OpenACC asynchronous kernels regions

In OpenACC 'kernels' decomposition, we're improperly nesting synchronous and
asynchronous data and compute regions, giving rise to data races when the
asynchronicity is actually executed, as is visible in at least on test case
with GCN offloading.

The proper fix is to correctly use the asynchronous interfaces, making the
currently synchronous data regions fully asynchronous (see also
<https://gcc.gnu.org/PR97390> "[OpenACC] 'async' clause on 'data' construct",
which is to share the same implementation), but that's for later; for now add
some more synchronization.

	gcc/
	* omp-oacc-kernels-decompose.cc (add_wait): New function, split out
	of...
	(add_async_clauses_and_wait): ...here. Call new outlined function.
	(decompose_kernels_region_body): Add wait at the end of
	explicitly-asynchronous kernels regions.
	libgomp/
	* testsuite/libgomp.oacc-c-c++-common/f-asyncwait-1.c: Remove GCN
	offloading execution XFAIL.

Co-Authored-By: Thomas Schwinge <thomas@codesourcery.com>