6502test: flesh out a little of the 6502 init paths
execve: remove some debug noise
6502: Start fleshing out how the port would work

Very incomplete at this point
lowlevel-z80: di/ei handling

Handle both di/ei restore cases don't assume we are just stacking disables
dragon: continue fleshing out

At this point it's a fairly complete build minus I/O devices but with some
guesswork on how we will deal with irq and syscall handling given the odd
memory mappings.

It does at least suggest that we can put it in cartridge + single task in
memory mode.