Restore 'GOMP_offload_unregister_ver' functionality

Restore 'GOMP_offload_unregister_ver' functionality

The recent commit 683f11843974f0bdf42f79cdcbb0c2b43c7b81b0
"OpenMP: Move omp requires checks to libgomp" changed the
'GOMP_offload_register_ver' interface but didn't change
'GOMP_offload_unregister_ver' accordingly, so we're no longer
actually unregistering.

	gcc/
	* config/gcn/mkoffload.cc (process_obj): Clarify 'target_data' ->
	'[...]_data'.
	* config/nvptx/mkoffload.cc (process): Likewise.
	libgomp/
	* target.c (GOMP_offload_register_ver): Clarify 'target_data' ->
	'data'.
	(GOMP_offload_unregister_ver): Likewise.  Fix up 'target_data'.