[libomptarget] Check for library with CUDA Driver API

That's what we really need to link the CUDA plugin against,
not the CUDA runtime API in CUDA_LIBRARIES! While the latter
comes with the CUDA SDK, the Driver API is installed with
the kernel driver and there is at most one per system. As
fallback we can use the stubs library distributed with the
CUDA SDK for linking.

Differential Revision: https://reviews.llvm.org/D42643

llvm-svn: 323787