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

[Tristan Ross via llvm-commits]

RossComputerGuy wrote:

> So this is an intentional decision by `nix` to not provide the whole tree?

Yeah, we just provided a minimal source tree.

> Does passing `-DLLVM_TOOL_LLVM_GPU_LOADER_BUILD=OFF` work?

Yes but this PR simplifies things by not having to manually provide it and run into the linker error.

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