FindCUDAToolkit: Understand NVHPC versioned math library layout

The NVHPC packages bundle the CUDA math libraries in a sibling
directory (`math_libs`) instead of in with the rest of the
cuda libraries.

Depending on the NVHPC package the math_libs folder can have
versioned subdirectories, therefore we prefer finding the
same versions as the CUDA Toolkit and falling back to the
latest when not possible.