[Libomptarget] Use binutils archive executable to address failing tests

Summary:
The static linking test ensures that we can statically link offloading
programs. To create the test we used `llvm-ar`. However, this may not
exist in the user's environment. This patch changes it to use the
binutils `ar` which should exist on every system running these tests
currently. In the future we should set up the dependencies properly.

openmp/libomptarget/test/offloading/static_linking.c

@@ -1,5 +1,5 @@
 // RUN: %libomptarget-compile-generic -DLIBRARY -c -o %t.o
-// RUN: llvm-ar rcs %t.a %t.o
+// RUN: ar rcs %t.a %t.o
 // RUN: %libomptarget-compile-generic %t.a && %libomptarget-run-generic 2>&1 | %fcheck-generic
 
 // UNSUPPORTED: nvptx64-nvidia-cuda-oldDriver