Moved from my flexio_t4 library.
Libary was updated to see if already defined before it now defines it.

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

Allow the user to call setTX with the one hardware pin, and hopefully allow them to turn on Open Drain mode or turn it back off.  I don't currently have any test cases that use this, so not sure how well it works.

Page 982.
Fixes https://forum.pjrc.com/threads/54711-Teensy-4-0-First-Beta-Test?p=196233&viewfull=1#post196233
- maybe this is needed for T3x, too? too late here to check this now.

The FlexIO registers are 32 bits, the masks were all setup for 16 bits so the high word of registers were not updated

warning: 'flexpwm' may be used uninitialized in this function [-Wmaybe-uninitialized]
warning: 'qtimer' may be used uninitialized in this function [-Wmaybe-uninitialized]

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?

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