Proper AVR preemptive interrupt handling (#10496)

Also simplify logic on all ARM-based interrupts. Now, it is REQUIRED to properly configure interrupt priority. USART should have highest priority, followed by Stepper, and then all others.

Marlin/src/HAL/HAL_LPC1768/HAL_timers.cpp

@@ -90,5 +90,4 @@ void HAL_timer_isr_prologue(const uint8_t timer_num) {
   }
 }
 
-
 #endif // TARGET_LPC1768