Add amd64 mdthread fields needed for the upcoming EFI RT exception
handling.

This is split into a separate commit from the main change to make it
easier to handle possible revert after upcoming KBI freeze.

Reviewed by: kevans
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Approved by: re (rgrimes)
Differential revision: https://reviews.freebsd.org/D16972