lib/systems: add emulator for mmix

[siraben]

Motivation for this change

Would be a good example to show when the emulators feature is documented (#106375). Try it out by creating a test.nix file at nixpkgs root with the following contents

{ pkgs ? import ./. {}}:
with pkgs;

let
  hello-cross = pkgsCross.mmix.hello;
  emul = pkgsCross.mmix.stdenv.hostPlatform.emulator pkgs;
in
rec {
  runit = writeScript "runit" ''
    #!/usr/bin/env sh
    ${emul} ${hello-cross}/bin/hello
  '';
}

then run

$ nix-build test.nix -A runit && ./result

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.