Full implementation of the Nemesis defense:
- Padding on IRQ;
- Storing and clearing all registers on IRQ;
- Restoring registers on RETI;
- Padding on RETI.

This way, the normal control flow access control logic can be used when
a RETI to PM is executed. Before, we used to store a single bit to
indicate that the previous instruction was a RETI to PM and the access
control logic would take that into account to allow jumps to non-entry
points.  However, this scheme is only secure with a single module. Also,
storing the previous PC more closely follows the model.

This allows us to evaluate the cost of the register backup separately
from the cost of the padding.