amd64: Define KVA regions for KMSAN shadow maps

KMSAN requires two shadow maps, each one-to-one with the kernel map.
Allocate regions of the kernels PML4 page for them. Add functions to
create mappings in the shadow map regions, these will be used by the
KMSAN runtime.

Reviewed by: alc, kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31295