[llvm] [LLVM] Fix llvm-gpu-loader use in standalone (PR #132664)

[Joseph Huber via llvm-commits]

https://github.com/jhuber6 commented:

So this is an intentional decision by `nix` to not provide the whole tree? If that's the case then we don't need to modify the CMake because it's changing the monorepo layout which is something we can't control. I feel like this is up to `nix` to fix. Does passing `-DLLVM_TOOL_LLVM_GPU_LOADER_BUILD=OFF` work?

https://github.com/llvm/llvm-project/pull/132664