thread0: Clear td_rux stats in proc0_post

proc0_post aims to reset the CPU usage accounting for all threads and
processes in the system to zero once the time of day is verified.
However, not all of the per-thread stats were not being cleared,
resulting in over-reported time for thread0 post-boot.

Reviewed by: olce, kib, markj
Fixes: bed4c5241663 ("Implement RUSAGE_THREAD. Add td_rux...")
Differential Revision: https://reviews.freebsd.org/D54040
(cherry picked from commit dafe50ebd874ca3ccd5908c441cee9f478c501f0)