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/sudolikeaboss/default.nix

@@ -22,10 +22,6 @@ buildGoPackage rec {
     fixDarwinDylibNames
   ];
 
-  postInstall = ''
-    install_name_tool -delete_rpath $out/lib -add_rpath $bin $bin/bin/sudolikeaboss
-  '';
-
   meta = with stdenv.lib; {
     inherit version;
     inherit (src.meta) homepage;