Fix merge of: amdgcn: fix up offload debug linking with LLVM 13

Fix merge of: amdgcn: fix up offload debug linking with LLVM 13

For some odd reasons (probably wrong merge conflict resolution),
one of the changes to config/gcn/mkoffload.c of
commit r11-9168-gcc84160c5f470b23b7aed4633f887df113b2675d
disappeared when merging origin/releases/gcc-11 into OG11.
Thus, apply I (re)applied it manually:

gcc/
	* config/gcn/mkoffload.c (main): Just let the attribute flags
	pass through.