Ensure that the returned status values are not ignored.  The old code was
not broken, but this is both safer and satisfies static analysis.

2020-04-23  Andrew Stubbs  <ams@codesourcery.com>

	PR other/94629

	libgomp/
	* plugin/plugin-gcn.c (init_hsa_context): Check return value from
	hsa_iterate_agents.
	(GOMP_OFFLOAD_init_device): Check return values from both calls to
	hsa_agent_iterate_regions.