'libgomp.c/target-43.c': '-latomic' for nvptx offloading

'libgomp.c/target-43.c': '-latomic' for nvptx offloading

... to avoid a regression with recent
commit 090f0d78f194e3cda23fe904016db77ea36c38fa
"openmp: Improve expand_omp_atomic_pipeline":

    unresolved symbol __atomic_compare_exchange_1
    collect2: error: ld returned 1 exit status
    mkoffload: fatal error: [...]/gcc/x86_64-pc-linux-gnu-accel-nvptx-none-gcc returned 1 exit status

	libgomp/
	* testsuite/libgomp.c/target-43.c: '-latomic' for nvptx offloading.

(cherry picked from commit 086bb917d6efa32d9841c34a3b762f4278c762cd)