treewide: get rid of platforms.allBut

Negative reasoning like `allBut` is a bad idea with an open world of
platforms. Concretely, if we add a new, quite different sort of
platform, existing packages with `allBut` will claim they work on it
even though they probably won't.

pkgs/development/libraries/cloog/0.18.0.nix

@@ -58,6 +58,6 @@ stdenv.mkDerivation rec {
        make[3]: *** [Box.lo] Error 1
 
     */
-    platforms = with stdenv.lib.platforms; allBut cygwin;
+    platforms = stdenv.lib.platforms.unix; # Once had cygwin problems
   };
 }

pkgs/development/libraries/cloog/default.nix

@@ -59,6 +59,6 @@ stdenv.mkDerivation rec {
        make[3]: *** [Box.lo] Error 1
 
     */
-    platforms = with stdenv.lib.platforms; allBut cygwin;
+    platforms = stdenv.lib.platforms.unix; # Once had cygwin problems
   };
 }