vm_phys: Add corresponding sysctl knob for loader tunable

The loader tunable 'vm.numa.disabled' does not have corresponding sysctl
MIB entry. Add it so that it can be retrieved, and sysctl -T will also
report it correctly.

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42138