[LLVM] Fix llvm-gpu-loader use in standalone

[RossComputerGuy]

Fixes the standalone builds of LLVM itself. In nixpkgs, we create a trimmed down copy of the LLVM monorepo for compatibility with older LLVM releases. We don't include libc in this copy which causes the llvm-libc-common-utilities target to not be defined. This causes llvm-gpu-loader to try linking to llvm-libc-common-utilities but fails due to the target not existing.

[jhuber6]

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?

[RossComputerGuy]

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.

[jhuber6]

I'm unsure, my gut feeling is that we shouldn't need to carry changes in LLVM just to make an external user's tree easier to use when there's an existing way to work around it. @nikic WDYT?