Clock needs to advance when interrupts are disabled

if interrupts are disabled, the result from `millis()` doesn't advance. The approach taken should be to check the `SCB_ICSR_PENDSTSET_Msk` bit and increment the tick count if it's set. This doesn't guarantee that `millis()` will advance as fast as it might if interrupts were enabled, but at least it will advance, and a tight loop based on `millis()` changing will terminate.