The code for T3.x was broken, especially for USB2...

And the code was not setup for T4.x.  So added it.

Code is setup that if there are no eventResponders that have registered to do interrupt calls, don't have the systic code in place to look for it.

Also yield code.  Now figures out if the user has any code that installs their own serialEventX methods, to call them, but if they have left the default weak pointer versions, to not call them in yield.  Likewise will only look for eventRespnder events if the sketch has actually attached any events.  If none of these conditions are true, yield will just check one flag and return.

T3.x Serial events cleanup

fixes #288

Recent additions did not update these counts.

Seperated T3.5 from T3.6 as PWM and Analog counts are different

With the new beta boards, for those users who don't need an SDCard for
their project, it would be great to allow them to use the 6 IO pins
associated with the SDCARD for other purposes.  This would probably
require a cheap adapter, but these are nice signals to be able to be
used.  They include:

SPI1: SCK, MISO, MOSI, CS0-2
Serail2: TX/RX
Serial4: TX/RX
I2C1: SCL/SDA

This change adds support for GPIO, SPI, Serial2 and Serial4

https://forum.pjrc.com/threads/17469-millis%28%29-on-teensy-3?p=104924&viewfull=1#post104924

systick_millis_count is already volatile, doing it twice is not needed.

thank you, duff !