Add support for pyb.micros() by using the systick timer.

I also removed trailing spaces from modpyb.c which affected a couple
of lines technically not part of this patch.

Tested using: https://github.com/dhylands/upy-examples/blob/master/micros_test.py

which eventually fails due to overflow issues (I could fix the test to compensate
but didn't bother)
If pyb.micros is going to be used in time critical Python code between disable_irq/enable_irq, then this function can't disable irq as well...