lib/systems: add emulator for mmix

lib/systems/default.nix

@@ -124,6 +124,8 @@ rec {
         then "${qemu-user}/bin/qemu-${final.qemuArch}"
         else if final.isWasi
         then "${pkgs.wasmtime}/bin/wasmtime"
+        else if final.isMmix
+        then "${pkgs.mmixware}/bin/mmix"
         else throw "Don't know how to run ${final.config} executables.";
 
     } // mapAttrs (n: v: v final.parsed) inspect.predicates