Implement SFX stage for musyx ucode.

With this patch, delay-based effects should work.
Basically, the SFX stage mixes up to 8 delayed subframes together and add the
resulting subframe to left and right subframes. Furthermore, a filtered version
of this subframe is used to update the sample history.

Games that use the SFX stage include:
The World is not Enough, Tarzan, Resident Evil 2, Polaris SnowCross

Musyx ucode implementation should now be complete !