Don't use libgomp 'cbuf' buffering with OpenACC 'async'

Don't use libgomp 'cbuf' buffering with OpenACC 'async'

The host data might not be computed yet (by an earlier asynchronous compute
region, for example.

	libgomp/
	* target.c (gomp_coalesce_buf_add): Update comment.
	(gomp_copy_host2dev, gomp_map_vars_internal): Don't expect to see
	'aq && cbuf'.
	(gomp_map_vars_internal): Only 'if (!aq)', do
	'gomp_coalesce_buf_add'.
	* testsuite/libgomp.oacc-c-c++-common/async-data-1-2.c: Remove
	XFAIL.

Co-Authored-By: Julian Brown <julian@codesourcery.com>