fixes #394

https://forum.pjrc.com/threads/57360-Teensy4-IntervalTimer-Channels-Problem

Edited here on the web - copied to local machine and it built as tested : https://forum.pjrc.com/threads/54711-Teensy-4-0-First-Beta-Test?p=209959&viewfull=1#post209959

Proposed add locations for  startup_early_hook(); and startup_late_hook(); just before two existing waiting while()'s.
With debug 'PRINT_DEBUG_STUFF' disabled the three T4B2's here hit the EARLY code about 1.2 ms and LATE at about 45.3 ms after the systick is started.

Where the current printf_debug_init() call is might be critical at some point adjust 'something' as you did enabling the Serial4 port for debug - before clocks and other things are started perhaps startup_reset_hook()

Comment out some of the main programs debug print statements plus add yield call to main loop.

yield can now call SerialEvent.

Put hack in that if the user has not overwritten the SerialEvent function, the weak linked one will turn off calling itself...

Saves power and prevents heating

Code from :: https://community.nxp.com/thread/389002
Name the fault dump info
New weak to override :: HardFault_HandlerC()

This fixes the PFDs for PLL2 - by default, it has totally wrong values. This sets the frequencies according to the diagram on Page 676.

The same fix is needed for PLL3 - I have code (same structure), but it does not work - Do have have any hints for me?
Is PLL3 even running? I not, can we start it? Or, if i is running - at which frequency, and  is there any non-std config?

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

..use NVIC_NUM_INTERRUPTS instead 
i know, very minor documentational issue :)