lm32: Skip last named param when computing save varargs regs

The cumulative args value in setup_incoming_varargs points at
the last named parameter. We need to skip over that (if present) to
get to the first anonymous argument as we only want to include
those anonymous args in the saved register block.

gcc/
	* config/lm32/lm32.cc (lm32_setup_incoming_varargs): Skip last
	named parameter when preparing to flush registers with unnamed
	arguments to th stack.