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/text/sift/default.nix

@@ -16,10 +16,6 @@ buildGoPackage rec {
 
   goDeps = ./deps.nix;
 
-  postInstall = lib.optionalString stdenv.isDarwin ''
-    install_name_tool -delete_rpath $out/lib -add_rpath $bin $bin/bin/sift
-  '';
-
   meta = with lib; {
     description = "sift is a fast and powerful alternative to grep";
     homepage = https://sift-tool.org;