Audio: implement YM2151 timers and IRQ support, add open bus behavior to IO space #143
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This PR adds the command line option
-enable-ym2151-irqwhich connects the YM2151 IRQ pin to the emulated CPU. It is disabled by default due to losing the audio render batching optimization.However, the timers will still work so the status can be polled even without the YM2151 IRQ pin connected.
For reads from $9Fxx space where nothing is responding, hardware returns $9F (due to the data bus still having the last byte of the operand from the instruction fetch). This behavior is now implemented here.