xorg: add meta.platforms

Add it to the generator script (generate-expr-from-tarballs.pl) and then
add the updated generated file (pkgs/servers/x11/xorg/default.nix) after
performing the steps outlined in the generator script.

NOTE: People seem to have updated .../default.nix manually (even though
it has a "THIS IS A GENERATED FILE.  DO NOT EDIT!" header), so I had to
skip some changes to not downgrade several packages.