makefile.odx: Build with more optimisations on the GCW Zero toolchain.
odx: Improve audio buffering, at the cost of a bit of lag.

When starting a game, buffer enough of it so that an underrun doesn't occur every 2 seconds.
Then refill the buffer in the same way if an underrun occurs so that more underruns don't occur.
odx: Use triple-buffering (on the GCW Zero) if SDL_TRIPLEBUF is defined.

This allows more CPU to be spent emulating frames, thus increasing FPS, instead of waiting for the secondary buffer to be free if a game is being emulated between 30 and 60 FPS and the CPU usage is not 100%.