Add pulse_width_percent 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
Hmm, it seems that this could overflow an int if the timer is 32-bit.  Could use long long multiplication, or divide the period by 100 first if the period is larger than 2^31/100.