19031: cpu/stm32/periph_timer: implement timer_set() r=benpicco a=maribu
### Contribution description
The fallback implementation of timer_set() in `drivers/periph_common` is known to fail on short relative sets. This adds a robust implementation.
### Testing procedure
Run `tests/periph_timer_short_relative_set` at least a few dozen times (or use https://github.com/RIOT-OS/RIOT/pull/19030 to have a few dozen repetitions of the test case in a single run of the test application). It should now succeed.
### Issues/PRs references
None
Co-authored-by: Marian Buschsieweke <marian.buschsieweke@ovgu.de>
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