MEDIUM: wdt: fall back to CLOCK_REALTIME if CLOCK_THREAD_CPUTIME is not available

MEDIUM: wdt: fall back to CLOCK_REALTIME if CLOCK_THREAD_CPUTIME is not available

At least FreeBSD has a fully functional CLOCK_THREAD_CPUTIME but it
cannot create a timer on it. This is not a problem since our timer is
only used to measure each thread's usage using now_cpu_time_thread().
So by just replacing this clock with CLOCK_REALTIME we allow such
platforms to periodically call the wdt and check the thread's CPU usage.
The consequence is that even on a totally idle system there will still
be a few extra periodic wakeups, but the watchdog becomes usable there
as well.