for micros() Use: ARM Synchronization Primitives, include of arm_math.h

Get systick_isr data safely without disable interrupts, uses systick_safe_read as indicator variable

micros() unchanged - minor systick_isr() diversion - one line added to systick_isr.

This saves a cycle - and checks both values to avoid _isr change of either value based on code/execution

Alternate micros() extends millis() using ARM_DWT_CYCCNT
> Also makes unused_interrupt_vector() weak to allow user override like Teensy3