haskell.compiler: unify logic determining whether NCG is available

[sternenseemann]

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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[sternenseemann]

Is a set rebuild for aarch64, see #379039 (comment)

[cdepillabout]

Nice refactoring!

[alexfmpe]

Do we no longer want the darwin bit in (stdenv.targetPlatform.isAarch64 && stdenv.targetPlatform.isDarwin) ?

[alexfmpe]

I assume there are more of these re-usable bits. Do we want a file for each or something like
useLLVM ? !((import ./common-utils.nix).have-ncg { inherit lib stdenv version; }),

[sternenseemann]

Do we no longer want the darwin bit in (stdenv.targetPlatform.isAarch64 && stdenv.targetPlatform.isDarwin) ?

Exactly, this was an incorrect assumption originally: 59ee081.

[sternenseemann]

I assume there are more of these re-usable bits. Do we want a file for each or something like useLLVM ? !((import ./common-utils.nix).have-ncg { inherit lib stdenv version; }),

Possibly yes, but I wouldn't want to commit to a design right now. There are multiple approaches we could take for further unifying these expressions. This one could also turn out to be special in a way, since it is one of the few things shared between the source built and binary GHCs.