Support Android on ARM

[kennytm]

Note: I have only tested to ensure all test cases in nix and mio works on Android 4.4 with ARM after this change. Other APIs not covered are not tested.

[carllerche]

Looks good, do you know if there is a way to setup CI for this environment?

[carllerche]

I merged here: c5bc332 but I would like to try to get CI somehow :)

[carllerche]

I filed an issue w/ Travis travis-ci/travis-ci#3376

[kennytm]

Thanks.

Travis may be able to fix their issue by providing the Android SDK + NDK and Android emulator / Genymotion simulator, but there are also works to do on the Rust side,

1. There is no Rust nightly for arm-linux-androideabi, so the CI will need to get a custom rustc somewhere (compiling it directly in Travis is inefficient, it may take hours) (this may help?)
2. Cargo does not support cross-testing yet (Add option to configure a test/binary runner rust-lang/cargo#1411), so we need to manually adb push the artifacts to the Android emulator to run them.

(I am more worried about how to setup a CI for iOS.)

[carllerche]

Closing in favor of: #97