[llvm] [offload][runtimes] Forward user-provided system configuration. (PR #96303)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

> > [3de162f](https://github.com/llvm/llvm-project/commit/3de162fee44376b3857c73958d656f76b17e50f5) sets it to `CUDAToolkit_BIN_DIR`, but the meaning it previously had (the path passed to `clang --cuda-path`) would be `CUDAToolkit_LIBRARY_ROOT`.
> 
> No, it takes the directory of `opt/cuda/bin` which is `opt/cuda` for example.

OK, it does `get_filename_component` of `CUDAToolkit_BIN_DIR`, but would be unnecessary since `CUDAToolkit_LIBRARY_ROOT` already contains that path. `CUDA_LIBDIR` is also the same as `CUDAToolkit_LIBRARY_DIR`. See https://cmake.org/cmake/help/latest/module/FindCUDAToolkit.html#result-variables

https://github.com/llvm/llvm-project/pull/96303