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/networking/assh/default.nix

@@ -9,9 +9,7 @@ buildGoPackage rec {
 
   nativeBuildInputs = [ makeWrapper ];
 
-  postInstall = stdenv.lib.optionalString (stdenv.isDarwin) ''
-    install_name_tool -delete_rpath $out/lib $bin/bin/assh
-  '' + ''
+  postInstall = ''
     wrapProgram "$bin/bin/assh" \
       --prefix PATH : ${openssh}/bin
   '';