darwin.xattr: replace with xattr from file_cmds

file_cmds 352.40.6 added a C-based implementation of xattr, which is the
version of xattr used on recent releases of macOS. Align nixpkgs with
what macOS is shipping, which has also the benefit of allowing xattr to
be cross-compiled (previously precluded due to Python limitations).

pkgs/top-level/darwin-packages.nix

@@ -221,7 +221,7 @@ impure-cmds // apple-source-packages // apple-source-headers // stubs // {
 
  trash = callPackage ../os-specific/darwin/trash { };
 
- xattr = pkgs.python3Packages.callPackage ../os-specific/darwin/xattr { };
+ inherit (self.file_cmds) xattr;
 
  inherit (pkgs.callPackages ../os-specific/darwin/xcode { })
  xcode_8_1 xcode_8_2