Make it possible to disable NUMA support with a tunable.

Make it possible to disable NUMA support with a tunable.

This provides a chicken switch for anyone negatively impacted by
enabling NUMA in the amd64 GENERIC kernel configuration.  With
NUMA disabled at boot-time, information about the NUMA topology
is not exposed to the rest of the kernel, and all of physical
memory is viewed as coming from a single domain.

This method still has some performance overhead relative to disabling
NUMA support at compile time.

PR:		231460
Reviewed by:	alc, gallatin, kib
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D17439