[llvm] [Offload] Add check-offload-unit for liboffload unittests (PR #137312)

[Callum Fare via llvm-commits]

callumfare wrote:

@jhuber6 Thanks for the review! Yeah it's still accurate, I updated it after the last change. It's still possible to restrict tests to a specific platform if you run the test binary manually with that flag, but the check-offload-unit target runs it on everything.

https://github.com/llvm/llvm-project/pull/137312