xtimer: set now pointer correctly in _update_short_timers() loop

This fixes `xtimer` to use `xtimer_now64()` instead of `xtimer_now()`
for updating the `*now` variable during the iteration in
`_update_short_timers()` function. The same function is used to
initialize `*now` in `_timer_callback()` below.

While using `xtimer_now()` in this iteration step does not hinder the
proper execution of all timers in the short term timers (for those the
`xtimer` module only looks at the `start_time` member, not the
`long_start_time` member) at least for the current long term time window
(I did not test higher cases), it sets the `long_start_time` member to 0
for all timers following in the list of timers after this iteration
step. However, external modules that rely on this to be correct,
e.g. evtimer [1], fail their calculations when trying to compare to
the current value to `xtimer_now64()`.

[1] https://github.com/RIOT-OS/RIOT/blob/11f3d68/sys/evtimer/evtimer.c#L118-L121

Co-Authored-By: Cenk Gündoğan <mail+dev@gundogan.net>

sys/xtimer/xtimer_core.c

@@ -292,7 +292,7 @@ static inline void _update_short_timers(uint64_t *now)
             /* assign new head */
             timer = timer_list_head;
             /* update current_time */
-            *now = _xtimer_now();
+            *now = _xtimer_now64();
         }
         else {
             timer->offset -= elapsed;