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/development/tools/continuous-integration/drone/default.nix

@@ -53,10 +53,6 @@ buildGoPackage rec {
     go generate github.com/drone/drone/store/datastore/ddl
   '';
 
-  postInstall = stdenv.lib.optionalString stdenv.isDarwin ''
-    install_name_tool -delete_rpath $out/lib -add_rpath $bin $bin/bin/drone
-  '';
-
   src = fetchFromGitHub {
     owner = "drone";
     repo = "drone";