'libgomp.c-c++-common/reduction-{5,6}.c': Restrict '-latomic' to nvptx offloading compilation

Fix-up for recent commit 33b64795
"OpenMP: Fix SIMT for complex/float reduction with && and ||"; see
commit d42088e4 "Avoid -latomic for amdgcn
offloading".

	libgomp/
	* testsuite/libgomp.c-c++-common/reduction-5.c: Restrict
	'-latomic' to nvptx offloading compilation.
	* testsuite/libgomp.c-c++-common/reduction-6.c: Likewise.