cores: Adds missing peripherals to m16/m32

> Port A not present on m16/m32 because PORTA was not defined in sim_megax.c for m16/32.
> Async external interrupt (INT2) missing on m16/32. Since unlike the others, this
  interrupt uses only 1 bit for sense control avr_extint.* were modified to support this
  new mode.
> As a consequence of the above, the dependency of sim_mega8.c on sim_megax.c was removed
  for simplicity.