Replace libextobjc submodule with a subtree

_Depends on jspahrsummers/xcconfigs#11._

Since libextobjc rarely changes, this saves clone/update time.

It also allows us to make modifications local to RAC. This PR (re-)namespaces the EXTScope cleanup function to avoid symbol conflicts if another framework exposes libextobjc symbols.

General-purpose instructions for updating the subtree can be found in the README of [jspahrsummers/objc-build-scripts](https://github.com/jspahrsummers/objc-build-scripts).