Add tests for others android platforms

[malbarbo]

The libc-test runs for arm-linux-android. It would be great if it were also run for aarch64-linux-android and i686-linux-android.

[malbarbo]

@alexcrichton I have a local git branch that add tests for these platforms. I had to update the android sdk from 24.4.1 to 25.2.5 (last version) to run aarch64 tests. Some tests are failing, I will try to fix them.

Would you accept such PR?

Should I have one container for each platform, or just one container? The container's size with the three platforms are about 15GB. Is it a problem?

[alexcrichton]

I'd love to add more platforms! PRs are of course always welcome :)

And yeah for now let's stick to a container-per-platform, and size shouldn't be a problem (I think the current image is huge)