Add i686-linux-android builds

[froydnj]

As Firefox looks to adding Rust support on all of its supported platforms, we'd like i686-linux-android (tier 3) to be given the same status as arm-linux-androideabi (tier 2). Part of that is having buildbot understand how to build binaries.

[alexcrichton]

I'll look into doing this once we get #99 in

[alexcrichton]

Fixed in e5ac28a, when #99 merges with the NDK updates I'll deploy and this'll make its way out