thread_create(): call cpu_copy_thread() after td_pflags is zeroed

By calling the function too early we might still have the td_pflags
value cached from the previous struct thread use. cpu_copy_thread()
depends on correct value for TDP_KTHREAD at least on x86.

Reported, bisected, and tested by: pho
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D36069