sound.c, sound.h: Decouple the cross-platform portion (mixing sound from the GBA channels) from the port-specific portion (output to the native audio hardware), adding some cross-platform service functions to sound.h. Fix a bug whereby & 0xFFFF and % 0xFFFF were sometimes interchangeably used, causing a desynchronisation between sound channels.

common.h: Add a new cross-platform function, ReGBA_AudioUpdate.