libgomp/gcn: Prepare for reverse-offload callback handling

libgomp/gcn: Prepare for reverse-offload callback handling

libgomp/ChangeLog:

	* config/gcn/libgomp-gcn.h: New file; contains
	struct output, declared previously in plugin-gcn.c.
	* config/gcn/target.c: Include it.
	(GOMP_ADDITIONAL_ICVS): Declare as extern var.
	(GOMP_target_ext): Handle reverse offload.
	* plugin/plugin-gcn.c: Include libgomp-gcn.h.
	(struct kernargs): Replace struct def by the one
	from libgomp-gcn.h for output_data.
	(process_reverse_offload): New.
	(console_output): Call it.