get parity w/ nukeykt/GenPlusGX Sound Emulation

As far as I know, the only thing emulation wise that Exodus lacks (if there are other things then work on those too definitely) compared to Genesis Plus GX in retroarch is ym2612 emulation accuracy.

They are using code from nukeykt at: https://github.com/nukeykt/Nuked-OPN2

Current repository: https://github.com/libretro/Genesis-Plus-GX