Get rid of most @rpath nonsense on Darwin

This requires some small changes in the stdenv, then working around the
weird choice LLVM made to hardcode @rpath in its install name, and then
lets us remove a ton of annoying workaround hacks in many of our Go
packages. With any luck this will mean less hackery going forward.

pkgs/tools/security/keybase/default.nix

@@ -16,10 +16,6 @@ buildGoPackage rec {
     sha256 = "1zgvriyir2ga0p4ah9ia1sbl9ydnrnw5ggq4c1ya8gcfgn8vzdsf";
   };
 
-  postInstall = stdenv.lib.optionalString stdenv.isDarwin ''
-    install_name_tool -delete_rpath $out/lib $bin/bin/keybase
-  '';
-
   buildFlags = [ "-tags production" ];
 
   meta = with stdenv.lib; {