Add support for emulating the MSP430 hardware multiply feature.

	* msp430-sim.c (sim_open): Do not allocate memory over the
	hardware multiply registers.
	(get_op): Add support for reads from the hardware multiply
	registers.
	(put_op): Add support for writes to the hardware multiply
	registers.
	(msp430_step_once): Add support for the RETI instruction used by
	the CPUX architecture.