Avoid -latomic for amdgcn offloading

libatomic isn't built for amdgcn but reduction-16.c adds it
via -foffload=-latomic when offloading for nvptx is enabled.
The following avoids linker errors when offloading to amdgcn is enabled
as well.

2021-04-21  Richard Biener  <rguenther@suse.de>

libgomp/
	* testsuite/libgomp.c-c++-common/reduction-16.c: Use -latomic
	only on nvptx-none.