Improve scheduler performance

Improve scheduler performance by flattening nonsensical topology layers
(layers with only one child don't serve any purpose).

This is especially relevant on non-AMD Zen systems after r322776. On my
dual core Intel laptop, this brings the kern.sched.topology_spec table down
from three levels to two.

Submitted by: jeff
Reviewed by: attilio
Sponsored by: Dell EMC Isilon