amd64: Avoid copying td_frame from kernel procs

When creating a new thread, we unconditionally copy td_frame from the
creating thread. For threads which never return to user mode, this is
unnecessary since td_frame just points to the base of the stack or a
random interrupt frame.

If KASAN is configured this copying may also trigger false positives
since the td_frame region may contain poisoned stack regions. It was
not noticed before since thread0 used a dummy proc0_tf trapframe, and
kernel procs are generally created by thread0. Since commit
df8dd6025af88a99d34f549fa9591a9b8f9b75b1, though, we call
cpu_thread_alloc(&thread0) when initializing FPU state, which
reinitializes thread0.td_frame.

Work around the problem by not copying the frame unless the copying
thread came from user mode. While here, de-duplicate the copying and
remove redundant re(initialization) of td_frame.

Reported by: syzbot+2ec89312bffbf38d9aec@syzkaller.appspotmail.com
Reviewed by: kib
Fixes: df8dd6025af8
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32057