Rename CCM_CCGR1_GPT defines

Current naming is confusing since it differs between GTP1 and GTP2. The GTP2 version is compatible to the naming in the data sheet. So I suggest to adjust the naming for  GPT1

As per the Teensyduino 1.48 release thread.

T3.x sketches could compile that had int mai() {} and not void() and setup():

So simply copied over setup.cpp from T3 folder of cores, and then changed setup.c to call main, which simply calls setup and loops calling loop/yield.

Added another endless loop after the call to main() to hang there if the users main returns.

Tested with simple sketch
```
#include <arduino.h>
int main (void) {
  pinMode(13, OUTPUT);
  for(;;) {
    digitalWriteFast(13, HIGH);
    delay(250);
    digitalWriteFast(13, LOW);
    delay(250);
  }
}
```
tried it on T3.6 verified before compile failed and now works.  Also compiled and run a normal app to just make sure it was not totally busted

Teensy4: Startup Hooks

fixes #394

Serial8 isn't a keyword now

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

Explicitly declare TMR channel registers in IMXRT_TMR_t

I suggest to explicitely delcare a (say)  IMXRT_TMR_CH_t struct which collects the registers for a TMR  channel instead of implicitely defining this struct in the declaration of IMXRT_TMR_t.

This would allow for an easier definition of user classes modelling a TMR channel.
Not a big deal, could also do that mayself the class code but it would be more convenient if this would be done in imxrt.h

Speed 24 MHz - not 24 Hz for Lowest voltage

CBT register for CANFD

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()

Added the ability to parametrize Teensy 3 core USB init delays

T4 pwm sdcard pins

The pins were not in the table before...

rx_pins should depend on cnt_rx_pins not cnt_tx_pins, currentlty the same so does not impact

T4 Serial5 support setTx and setRx to pins 38, 39

Updated to get A12/A13 operational