amdgcn: Tune default OpenMP/OpenACC GPU utilization

	libgomp/
	* plugin/plugin-gcn.c (parse_target_attributes): Automatically set
	the number of teams and threads if necessary.
	(gcn_exec): Automatically set the number of gangs and workers if
	necessary.

Co-Authored-By: Andrew Stubbs <ams@codesourcery.com>