Allow empty NUMA memory domains to support Threadripper2

The AMD Threadripper 2990WX is basically a slightly crippled Epyc.
Rather than having 4 memory controllers, one per NUMA domain, it has
only 2 memory controllers enabled. This means that only 2 of the
4 NUMA domains can be populated with physical memory, and the
others are empty.

Add support to FreeBSD for empty NUMA domains by:

• creating empty memory domains when parsing the SRAT table, rather than failing to parse the table
• not running the pageout deamon threads in empty domains
• adding defensive code to UMA to avoid allocating from empty domains
• adding defensive code to cpuset to avoid binding to an empty domain Thanks to Jeff for suggesting this strategy.

Reviewed by: alc, markj
Approved by: re (gjb@)
Differential Revision: https://reviews.freebsd.org/D1683