SCP: Allow the VM to print simulator stop message information in lieu of, or in addition to, the default message.

The current implementation of "run_cmd" in scp.c calls "fprint_stopped_gen" (via "fprint_stopped") to print the message associated with the "sim_instr" return status.  Messages associated with VM stops must be provided to the SCP via the "sim_stop_messages" array.

"fprint_stopped_gen" prints the status message in a rigid format: the message string, a comma, the program counter register name, a colon, the current PC value, and the instruction at that address in symbolic format.

For example:

       HALT instruction, P: 24713 (LDA 1)

Only the message string is under the control of the VM.  If additional information is needed, it can only be added before the first comma.

The HP2100 simulator does this for halt instructions, which contain device select code and flag hold/clear bit fields that, in practice, are used to communicate to the operator the significance of the particular halt encountered, rather than to affect the device interface:

       HALT instruction 102077, P: 24713 (LDA 1)

To implement this, the simulator must define the message as a variable and then copy the formatted octal value into the buffer at the appropriate location before returning from "sim_instr".

However, if the VM wants to display a different register value, e.g.:

       Self test #13 complete, STAT: 000020

...this cannot be done without also displaying the program counter, which may be irrelevant for the given stop condition.