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/servers/monitoring/prometheus/node-exporter.nix

@@ -14,10 +14,6 @@ buildGoPackage rec {
     sha256 = "0rm43jjqv7crfahl973swi4warqmqnmv740cg800yvzvnlp37kl4";
   };
 
-  preFixup = stdenv.lib.optionalString stdenv.isDarwin ''
-    install_name_tool -delete_rpath $out/lib $bin/bin/node_exporter
-  '';
-
   # FIXME: megacli test fails
   doCheck = false;