Simplify interrupt and RTOS timer tick handlers

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.

FreeRTOS/Source/portable/esp8266/port.c

@@ -198,6 +198,8 @@ portBASE_TYPE xPortStartScheduler( void )
     }
 
     /* Initialize system tick timer interrupt and schedule the first tick. */
+    _xt_isr_attach(INUM_TICK, sdk__xt_timer_int);
+    _xt_isr_unmask(BIT(INUM_TICK));
     sdk__xt_tick_timer_init();
 
     vTaskSwitchContext();