amd64: expicitly re-init td_frame in copy_thread()

Otherwise we are using whatever the value was left from the previous
thread run on kernel entry from usermode. Typically it would be the
desired value as is, but it is not guaranteed.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D35888