lib.systems: introduce toolchain, cc, and bintools attributes

[RossComputerGuy]

Things done

Replaces useLLVM, useArocc, and useZig with toolchain, cc, linker, and bintools attributes. This might not produce any rebuilds but we'll see. This has the advantage of preventing using${compiler} flags from colliding and not working correctly if we were to stack multiple pkgs* together.

• 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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[tomberek]

The main idea and motivation is in: pkgs/stdenv/cross/default.nix

[philiptaron]

I like the direction this is going in -- if there's an enum, make it be an enum, not a set of booleans.

I'd appreciate a "next steps" portion of this PR, where you give some guidance on how and what's going to happen next, especially if there are breaking changes.

[RossComputerGuy]

I'd appreciate a "next steps" portion of this PR

Next step is to add exports for LD or change exports for LD to the linker explicitly set. The idea is the bintools and linker can be less connected. From there, it'll probably be more explicit defining of the CC and other tools. Maybe a setup hook inside CC wrapper or other stuff could do it. Then I'll redo my CPU model PR and make some of that optimization stuff more flexible between different compilers.

where you give some guidance on how and what's going to happen next, especially if there are breaking changes.

Probably any other things we could look into doing that makes it easier to add an LLVM bootstrap to Linux.