[Libomptarget] Only build GPU tests if a GPU is found on the system

    Currently we build tests as long as the libraries are found on the
    machine. This doesn't necessarily mean there is a GPU to use though.
    This patch changes it to where we only will build the tests if we found
    a compatible GPU via `nvptx-arch` or `amdgpu-arch`.

    The only downside to this I could see if someone were to build LLVM on a
    home node of a cluster and then wished to run the tests after switching
    to a compute node. For this I think we should allow it to be overridden.
    I think that's better than allowing us to run tests that will fail by
    default.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D142018