Make the number of interrupt vectors configurable

[rhgndf]

Some modern 8051 microcontrollers have a lot of ISRs. STC8H8K64U for example has 28. Hopefully this PR should be a simple enough one that isn't intrusive at all.
Would be an interesting idea to see if this allows porting of libraries like TinyUSB to it.

[cyrozap]

Thanks for the PR. IIRC, the reason I wrote the code in 99d5a71 the way I did was to make it straightforward to either add some indicator as to which 8051 interrupt source triggered the RISC-V interrupt or to implement a custom interrupt handler in the 8051 code (since handling it in the emulated RISC-V might be to slow for some interrupts). If we're going to abandon that, it would probably be better to make the macro emit setb misc_bit0__new_interrupt; reti directly instead of an ljmp to a common setb misc_bit0__new_interrupt; reti, just to avoid the indirection. If you make that change, I'll merge this PR.

Probably the long term strategy should be to do what you suggested in #9, since right now there's no way for the RISC-V code to know which interrupt was fired without checking literally every interrupt source on the microcontroller.