libffcall: mark not broken on darwin

Not sure what the issue was, but now it just works.

pkgs/development/libraries/libffcall/default.nix

···
     homepage = "https://www.gnu.org/software/libffcall/";
     license = licenses.gpl2Plus;
     platforms = platforms.unix;
-    # never built on aarch64-darwin since first introduction in nixpkgs
-    broken = stdenv.isDarwin && stdenv.isAarch64;
   };
 }