Add recent pthread_internal_t fields to private loader with dynamic dispatch on Android version

[derekbruening]

For the port to Android in #1701 we had to emulate the Android loader in order to load a private copy of Bionic for C++ and other clients who want to use libc. Unfortunately as part of that we have to depend on the precise pthread_internal_t structure used by Android.

It looks like the structure has changed. We'll need to add dynamic dispatch to handle varying Android versions.