Set td_oncpu before dropping the thread lock during a switch.

After r355784 we no longer hold a thread's thread lock when switching it
out. Preserve the previous synchronization protocol for td_oncpu by
setting it together with td_state, before dropping the thread lock
during a switch.

Reported and tested by: pho
Reviewed by: kib
Discussed with: jeff
Differential Revision: https://reviews.freebsd.org/D23270