sim_interupts: simplification of interrupt handling

changes interrupt handling use of fifo to a more simple and less
 complex bitmap approach.  for interrupt intensive simulations this
 can save hundres of cycles per interrupt.

the fifo approach touches memory to sort and check the queue,
 dependent on number of interrupts in queue...  using bitmaps
 setting and clearing interupts is more alu dependant ie. in
 register/processor and relatively independant of number of
 interrupts...  the ffs() function, or find first set, is used
 since this may be tailored to use processor features available
 relative to a native build.

	modified:   simavr/sim/run_avr.c
	modified:   simavr/sim/sim_interrupts.c
	modified:   simavr/sim/sim_interrupts.h