Address pushed by BRK (and presumably interrupts) onto stack is too high by 1

[gardners]

BRK, RTI and interrupts must all use the same semantics for the machine to run.
They do, but the pushed address is too high by one.

[gardners]

Need to:

1. Reduce by 1 the PC pushed by BRK.
2. Reduce by 1 the PC pushed by interrupts.
3. Increment PC pulled when calling RTI (this makes it the same as RTS in fact)

[gardners]

Implemented the fix, and confirmed that the right values get written on the stack, and C64 ROMs start up fine with it. However, the BRKN stress test still complain that the wrong data gets written to the stack. Not sure what exactly is wrong now.

[gardners]

Fixed wrong way. Should be PC+1, not PC-1. Fixed in commit f5dd623, which will hopefully fix this issue.

[gardners]

Fixed.