Merge 'staging' into closure-size

- there were many easy merge conflicts
- cc-wrapper needed nontrivial changes

Many other problems might've been created by interaction of the branches,
but stdenv and a few other packages build fine now.

pkgs/tools/compression/gzip/default.nix

@@ -31,6 +31,8 @@ stdenv.mkDerivation rec {
         is just a bonus.
       '';
 
+    platforms = stdenv.lib.platforms.all;
+
     license = stdenv.lib.licenses.gpl3Plus;
   };
 }