Add support for timers with bitness less of full type.

For Cortex-M 24-bit SysTick for example.

TODO: Work on "width" terminilogy for main type...
Update for changed order of ITimer template params.
Add Cortex-M SysTick support.
Allow target to override OBJCOPY/OBJDUMP.
Reference cortext-uni-startup from top PTL dir.
Add x86 target, mostly for unit testing.
Add unit tests dir.