libao audio driver

This adds support for audio output with [`libao`](https://www.xiph.org/ao/) library.

The main reason for this is to have audio output on OpenBSD, as it's `sndio` audio API isn't supported by any of the already implemented audio drivers in Godot. Besides `sndio` there are a few more plugins in `libao`: ALSA, OSS, PulseAudio, Sun.

As for now, the dirver doesn't bother to read any `GLOBAL_DEF` for number of channels (only stereo) or mix rate (hardcoded to 44100).

ALSA driver was used as a reference.