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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

> CUDA_TOOLKIT_ROOT_DIR

 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`.

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