Add initial memory locality cost awareness to the VM, and include
a basic ACPI SLIT table parser.

For now this just exports the map via sysctl; it'll eventually be useful
to userland when there's more useful NUMA support in -HEAD.

• Add an optional mem_locality map;
• add a mapping function taking from/to domain and returning the relative cost, or -1 if it's not available;
• Add a very basic SLIT parser to x86 ACPI.

Differential Revision: https://reviews.freebsd.org/D2460
Reviewed by: rpaulo, stas, jhb
Sponsored by: Norse Corp, Inc (hardware, coding); Dell (hardware)