[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?

[nikic]

@jhuber6 The standalone build for LLVM is largely expected to work out of the box, as documented in https://llvm.org/docs/GettingStarted.html#stand-alone-builds. It lists which subprojects llvm can depend on (llvm, cmake, third-party) and only those should be necessary. We could of course add libc to that list, but making that a requirement seems of dubious value at this point. So I do think we should be catering to that use case.

I'm somewhat skeptical about the original change itself though (left a comment at #132096 (comment)). There was some discussion about adding a dependency on libc at some point to provide FP libcall implementations, but I didn't really expect it to be added for this reason and at this point in time (with libc's current maturity level, such as inability to be built for key platforms).