Motivation for this change

http://robert.ocallahan.org/2017/09/rr-50-released.html

Not mentioned there, and important to me and at least a few other NixOS users I know,
is that this adds support for Kaby Lake processors:

http://robert.ocallahan.org/2017/09/rr-50-released.html

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.