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/shells/elvish/default.nix

@@ -4,10 +4,6 @@ buildGoPackage rec {
   name = "elvish-${version}";
   version = "0.10";
 
-  postInstall = stdenv.lib.optionalString (stdenv.isDarwin) ''
-    install_name_tool -delete_rpath $out/lib $bin/bin/elvish
-  '';
-
   goPackagePath = "github.com/elves/elvish";
 
   src = fetchFromGitHub {