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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> CUDA_TOOLKIT_ROOT_DIR is actually deprecated but still used and required by offload/list/lit.site.cfg.in.

We should probably fix that.

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