Spinning for the correct time has the side effect that it may cause
infinite recursion if the callback function calls timer_set.
timer_set->callback->...->timer_set->callback->...->timer_set-> infinity
In theory, the drawback is that the callback for very short timeouts
(<2 lptmr ticks) may be delayed up to 2 lptmr ticks (61 µs)
In practice however, tests performed using tests/bench_timers shows that
this change only affects the accuracy of the timer target when timer_set
is called with a timeout of 0, which results in a delay of 30 µs.
Uses timer freerunning counter mode (TFC) and updating an internal
reference point instead of relying on RTT for reference time. The
target accuracy has been greatly improved for all test cases in
bench_periph_timer
