used in the kernel only (by virtue of checking for _KERNEL),
ports like lsof (part of gtop) cheat. It sets _KERNEL, but does
not set either AIM or E500. As such, PCPU_MD_FIELDS didn't get
defined and the build broke.
The catch-all is to define PCPU_MD_FIELDS with a dummy integer
when at the end of line we ended up without a definition for it.