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/applications/networking/syncthing/inotify.nix

@@ -25,8 +25,6 @@ buildGoPackage rec {
     substitute $src/etc/linux-systemd/user/syncthing-inotify.service \
                $bin/lib/systemd/user/syncthing-inotify.service \
                --replace /usr/bin/syncthing-inotify $bin/bin/syncthing-inotify
-  '' + stdenv.lib.optionalString stdenv.isDarwin ''
-    install_name_tool -delete_rpath $out/lib -add_rpath $bin $bin/bin/syncthing-inotify
   '';
 
   meta = with stdenv.lib; {