6502: Fix ZP = 0 assumptions and note further fixes needed

6502: Fix ZP = 0 assumptions and note further fixes needed

The runtime for cc65 assumes it can save to ZP and jump to the same address
via a 16bit index. That fails on our 65C816 build and also on things like the
HuC. Also fix the same offence in ct0.s

Note that we need to save jmpvec+1/2 for re-entry (CC65 forgets to mention this)
and also that we can't just swap zp because unlike a kernel/IRQ switch where
this is valid in Fuzix it's possible to take one signal in another, thus
we must stack the ZP state somewhere (at 20 odd bytes probably the C stack)