lto-wrapper.cc: Add offload target name to 'offload_args' suffix

lto-wrapper.cc: Add offload target name to 'offload_args' suffix

lto-wrapper.cc's compile_offload_image calls mkoffload with
an @./a.offload_args argument ('a.' in case of, e.g., 'a.out'). However,
when generating code for both nvptx and gcn, they use the same name
with -save-temps. Hence, this commit adds a <target> + '.' before
'offload_args' in line with other offload-target-specific files.

gcc/ChangeLog:

	* lto-wrapper.cc (compile_offload_image): Prefix 'offload_args'
	suffix by the target name.