[libomptarget-nvptx-bc] Pass found CUDA installations

We already know where the CUDA SDK is, so there is no point in
letting Clang search for it again and possibly finding no or
a different installation.

--cuda-path is supported since the beginning of CUDA support in
Clang, so making this required doesn't impose additional restrictions.

Differential Revision: https://reviews.llvm.org/D46930