Found when working with -save-temps and looking at 'mkoffload'
with a GCC configured for both nvptx and gcn offloading.

Before (for 'a.out') for mkoffload:a.offload_args now: a.amdgcn-amdhsa.offload_args 
and a.nvptx-none.offload_args
OK for mainline?

Tobias

PS: The code does not free the 'xmalloc'ed memory, but that's also
the case of all/most 'concat' in this file; the concat could also
be skipped when no save_temps is used, in case this optimization
makes sense.