RTOS Timer tick handler is now the same as any other ISR.
This causes a few subtle behaviour changes that seem OK but are worth noting:
* RTOS tick handler sdk__xt_timer_int() is now called from one stack
frame deeper (inside _xt_isr_handler()), whereas before it was called
from the level above in UserHandleInterrupt. I can't see any way that
the extra ~40 bytes of stack use here hurt, though.
* sdk__xt_timer_int() was previous called after all other interrupts
flagged in the handler, now it's called before the TIMER FRC1 & FRC2
handlers. The tick handler doesn't appear to do anything particularly
timing intensive, though.
* GPIO interrupt (value 3) is now lower priority than the SPI
interrupt (value 2), whereas before it would have been called before
SPI if both interrupts triggered at once.
