Define MCU in one place at the top of the file.  Also allow it to be
overridden by the command line to compile for a different MCU without
changing the Makefile.
Reduce confusion, remove -nostdlib as it isn't obvious that it is
only being passed when compiling (which isn't linking so doesn't use
it), and isn't being passed to the linker (where it would fail if it
was, because routines from the library are used).

https://forum.pjrc.com/threads/31233-float-and-double?p=86860&viewfull=1#post86860

Per forum
https://forum.pjrc.com/threads/1227-Teensy3-alternative-for-dtostrf()?p=86671&viewfull=1#post86671

In case the desired frequency is too low to be obtained with F_TIMER
because prescale and mod were at their limits, switch automatically to
the alternative clock source (31250Hz) before determining prescale and
mod with the same algorithm as before.

50.6.2 SCAN Control register

Bug found by Chronologist, https://forum.pjrc.com/threads/29624-Using-TSI-for-capacitance-measurement

https://forum.pjrc.com/threads/29446-Teensy-Hardware-Flow-Control-RTS-CTS?p=80425#post80425

can now receive and send Midi Time Codes
and send Real Time messages

Since newlib 2.2.0 itoa() and utoa() are declared in stdlib.h and redefining them makes the compiler unhappy

https://forum.pjrc.com/threads/28932-LC-is-10-9-times-slower-than-T3-1?p=76072&viewfull=1#post76072

https://forum.pjrc.com/threads/25153-PDB0CH1-register-definitions

https://forum.pjrc.com/threads/28932-LC-is-10-9-times-slower-than-T3-1?p=75775&viewfull=1#post75775

https://forum.pjrc.com/threads/28932-LC-is-10-9-times-slower-than-T3-1

https://forum.pjrc.com/threads/28325-Teensy-LC-and-delay

or, alternativly one could write size_t strlen(const char *s) {return __builtin_strlen(s);} to use the gcc-builtin function.
I think, both are faster than the bytewise search.

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