Add pulse_width_ratio to teensy.

While testing pulse_width_ratio, I discovered that trying to do any floating point opertions on the teensy would cause a hang. This is because the teensy CPU has no HW floating point support.

Fix stmhal and teensy print routines to report actual prescaler and period.
Fix teensy build to use soft-float
Add USE_ARDUINO_TOOLCHAIN option to teensy build