mkoffload: Cleanup temporary omp_requires_file

The file (suffix ".mkoffload.omp_requires") used to save the 'omp requires'
data has to be passed to maybe_unlink for cleanup or -v -save-temps stderr
diagnostic. That was missed before. - For GCN, the same has to be done for
the files with suffix ".mkoffload.dbg.o".

gcc/ChangeLog:

	* config/gcn/mkoffload.cc (main): Add omp_requires_file and dbgobj to
	files_to_cleanup.
	* config/i386/intelmic-mkoffload.cc (prepare_target_image): Add
	omp_requires_file to temp_files.
	* config/nvptx/mkoffload.cc (omp_requires_file): New global static var.
	(main): Remove local omp_requires_file var.
	(tool_cleanup): Handle omp_requires_file.