(REG_MDR): Define.
* simops.c: Implement "cmp", "calls", "rets", "jmp" and
a few additional random insns.
We can now function calls. We get out of crt0 into main now, then lose
when calls are nested (because don't handle movm yet).
(REG_D0, REG_A0, REG_SP): Define.
* simops.c: Implement "add", "addc" and a few other random
instructions.
Starting to simulate instructions for the mn10300. Executes some of
the crt0 code now!
