Specify target and linker for tests

Author: DavidTruby

flang/test/Driver/atomic.f90

@@ -1,5 +1,5 @@
-!RUN: %flang -fopenmp -rtlib=libgcc -### %s 2>&1 | FileCheck --check-prefixes=GCC %s
-!RUN: %flang -fopenmp -rtlib=compiler-rt -### %s 2>&1 | FileCheck --check-prefixes=CRT %s
+!RUN: %flang --target=aarch64-unknown-linux-gnu -fuse-ld=ld -fopenmp -rtlib=libgcc -### %s 2>&1 | FileCheck --check-prefixes=GCC %s
+!RUN: %flang --target=aarch64-unknown-linux-gnu -fuse-ld=ld -fopenmp -rtlib=compiler-rt -### %s 2>&1 | FileCheck --check-prefixes=CRT %s
 
 !GCC: -latomic
 !CRT-NOT: -latomic