threadsCross: factor out copy-and-pasted code

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Description of changes

In #236598 a chunk of code was simply copy-pasted-tweaked. The more often this happens, the more unmaintainable nixpkgs becomes. Let's use the wonderful functions and conditionals that Nix gives us to avoid this duplication.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• 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/)
• 23.11 Release Notes (or backporting 23.05 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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Verified no change to eval for pkgsCross.mingwW64.buildPackages.{gcc48 gcc49 gcc6 gcc7 gcc8 gcc9 gcc10 gcc11 gcc12 gcc13 go_1_18 go_1_19 go_1_20}