[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.