pending interrupts.
* interrupts.c (interrupts_process): Keep track of the last number
of masked insn cycles.
(interrupts_initialize): Clear last number of masked insn cycles.
(interrupts_info): Report them.
(interrupts_update_pending): Compute clear and set masks of
interrupts and clear the interrupt bits before setting them
(due to SCI interrupt sharing).
* interrupts.h (struct interrupts): New members last_mask_cycles
and xirq_last_mask_cycles.
