Revert part of the r283303 (by jhb):

Revert MFC of r270223, which bumped MAXCPU on amd64 from 64 to 256.
The cpuset_getaffinity(2) and cpuset_setaffinity(2) check minimum set
size, which now fails for binaries compiled on 10.0 with MAXCPU == 64.

Submitted by: jhb
PR: 200802