Add support for pins 26 and 27

Fix for crash @ 48 & 24MHz on K66

On Serial 3 this addition is for all 3.x.  Before it was ifdefed to only
do work on Teensy_LC as there is only one valid TX pin.  But we now pass
in open drain as an option so this code should allow you to not turn it
on/off on Serial3 on all of these boards

and on Serial4 and Serial5 on the new boards.

Updated Serial2.
It had valid TX for T3.2 of 10 and 31, but 31 on T3.5 is RX4

Likewise RX for T3.2 of 9 and 26, but on T3.5 26 is RX1

fixes 136

fixes #138

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

fixes #131

Corrected typecast in usb_flightsim.cpp

Signed-off-by: Jorg Neves Bliesener <jbliesener@bliesener.com>

fixes #130

Teensy 2 - Serial1.write() - maybe bypass queue

Remove possible race condition in setting and clearing transmitting.
Not from direct handling of this interrupt, but maybe another interrupt
happens right after we set UDR1, which by the time it finishes, the
transmit complete interrupt happens which clears it and returns and we
set transmitting.

add compare and llwu register defs

Add Serial.clear() to Teensy 3.x

Add same speedup from Arduino core, that on Serial1.write, check to see
if output queue is empty and data register empty, if so simply stuff the
character out in the output queue, without needing to use interrupt to
start transmission.  Helps speed up fast IO like 1MBS

Also, changed how UCSR1B is updated, to be like Arduino.  In particular
it turns on and off specific bit(s) instead of complete set of register.
Needed if you wish to emulate half duplex with this uart.

On Teensy 3.x - we have Serial1.clear, Serial2.clear...

But not Serial.clear.

Serial.clear() exists for Teensy2, so thought I would try to add it for
Teensy 3s...

Allow the teensy3 library to be used with strict C++11 conformance.

Add volume/mute feature to AudioInputUSB