amd64: Add kexec support

The biggest difference between this and arm64 kexec is that we can't
disable the MMU for amd64, we have to instead create a new "safe" page
table that the trampoline and "child" kernel can use. This requires a
lot more work to create identity mappings, etc.

Reviewed by: kib
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D51623