OpenDingux optimisations

I attach 3 commits that optimise the OpenDingux build of MAME4ALL:

* One of the commits is aimed at reducing audio underruns causing crackling, by buffering a bit more of the audio at once. This introduces a bit of lag in the audio (1 SDL buffer's worth, which is about 20 milliseconds).
* One of the commits is aimed at requesting triple-buffering on OpenDingux for the GCW Zero. It allows some games that run between 30 and 60 FPS, which display at 30 FPS with double-buffering, to emulate and display, as well as synthesise audio for, more frames per second.
* One of the commits builds MAME4ALL with -O3 and default toolchain flags instead of being explicit about MIPS-specific flags. This allows the toolchain's optimisations to evolve.