Create the libreactnative.so dependency (#43908)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/43908

This creates a new dynamic library that we want to be the only .so that is loaded from apps/libraries.

Changelog:
[Internal] [Changed] - Create the libreactnative.so dependency

Reviewed By: cipolleschi

Differential Revision: D55751682

fbshipit-source-id: 50f4167dc2f9953a8673b28dba3357e19fe88d6b