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/filesystems/go-mtpfs/default.nix

@@ -16,9 +16,5 @@ buildGoPackage rec {
     sha256 = "1jcqp9n8fd9psfsnhfj6w97yp0zmyxplsig8pyp2gqzh4lnb5fqm";
   };
 
-  preFixup = stdenv.lib.optionalString stdenv.isDarwin ''
-    install_name_tool -delete_rpath "$out/lib" $bin/bin/go-mtpfs
-  '';
-
   goDeps = ./deps.nix;
 }