[arch][arm-m] refactor systick code into separate module

Add generic support for running systick in monotonic mode and providing
current_time() routines.

Platforms may choose not to use this, thus it is moved into an optional
module.
[platform][stellaris] convert to generic systick mechanism
[platform][lpc15xx] convert to new generic systick timer mechanism
[platform][stm32f1xx/stm32f2xx/sam3] convert to new generic systick module