FindCUDAToolkit: Add CUDAToolkit_LIBRARY_ROOT

On scattered installations version.txt and nvvm are located at this location.
This may be useful to users and will allow us in the future to parse
version.txt instead of invoking nvcc to figure out the CUDA toolkit version.

We also add it to CMakeDetermineCUDACompiler in preparation for future use by
Clang code.
3 files changed