gcn/mkoffload: Cleanup temporary dbgobj file

gcn/mkoffload: Cleanup temporary dbgobj file

The file (suffix ".mkoffload.dbg.o") used to save the dbgobj data
data has to be passed to maybe_unlink for cleanup or -v -save-temps stderr
diagnostic. That was missed before.

This is a partial backport of commit r13-2125, "mkoffload: Cleanup
temporary omp_requires_file", only for GCN's mkoffload and its dbgobj
file as 'omp requires' is not supported on GCC 12 and, hence,
omp_requires_file does not exist on this branch.

gcc/ChangeLog:

	* config/gcn/mkoffload.cc (main): Add dbgobj to files_to_cleanup.

(cherry picked from commit 713ec97e593bd4d9915a13bc4047f064fec0e24a)