pkgsCross.x86_64-unknown-redox: fix evaluation of stdenv

pkgs/development/libraries/relibc/default.nix

···
     description = "C Library in Rust for Redox and Linux";
     license = licenses.mit;
     maintainers = [ maintainers.aaronjanse ];
-    platforms = platforms.redox;
+    platforms = platforms.redox ++ [ "x86_64-linux" ];
   };
 }