amd64: Clear the local TSS when creating a new thread

Otherwise it is copied from the creating thread. Then, if either thread
exits, the other is left with a dangling pointer, typically resulting in
a page fault upon the next context switch.

Reported by: syzkaller
Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30607