Enable KPTI by default on amd64 for non-AMD CPUs

Kernel Page Table Isolation (KPTI) was introduced in r328083 as a
mitigation for the 'Meltdown' vulnerability. AMD CPUs are not affected,
per https://www.amd.com/en/corporate/speculative-execution:

We believe AMD processors are not susceptible due to our use of
privilege level protections within paging architecture and no
mitigation is required.

Thus default KPTI to off for AMD CPUs, and to on for others. This may
be refined later as we obtain more specific information on the sets of
CPUs that are and are not affected.

Submitted by: Mitchell Horne
Reviewed by: cem
Relnotes: Yes
Security: CVE-2017-5754
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D13971