llvm: fix crossbuilding on Darwin

[brndnmtthws]

The --build-id=sha1 flag isn't supported on Darwin, and we need to check that neither the host or build platform is Darwin.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[alyssais]

Is there no way to specify separate link flags for the build and host platforms? The comment above sounds like it would be suboptimal to produce builds for Linux hosts that didn't use --build-id.

[brndnmtthws]

I'm not entirely sure, but I think it might make sense to drop the check for hostPlatform here. If you're cross-compiling from Linux, it would be fine to include the build flag (i.e., the compiler is being build to run on Linux, but produce binaries for Darwin).

I think you should be able to compile binaries for macOS from Linux, but I haven't tried this yet.

[alyssais]

It's not something you can currently do in Nixpkgs. Checking hostPlatform seems right to me, because it's the host linker that needs to support the argument. Checking the build linker seems like a workaround for not being able to distinguish between build and host flags, which I bet there's a way to do.