Update td_runtime of running thread on each statclock().

Normally td_runtime is updated on context switch, but there are some kernel
threads that due to high absolute priority may run for many seconds without
context switches (yes, that is bad, but that is true), which means their
td_runtime was not updated all that time, that made them invisible for top
other then as some general CPU usage.

MFC after: 1 week
Sponsored by: iXsystems, Inc.