x86: Harmonize GENERIC and MINIMAL

• Reorder MINIMAL so everything is in the same order as in GENERIC.

• Wherever comments diverged, except for the explanatory comment at the top, copy the GENERIC version to MINIMAL.

• Add KDTRACE_FRAME to i386 GENERIC; it was already in MINIMAL, and adding it to GENERIC seemed like the more correct move.

With these changes, MINIMAL is a strict subset of GENERIC, apart from
the identifier and the explanatory comment at the top.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D57729

(cherry picked from commit 01c8644093c28d09f0d41fb522dc53f1ab696a7e)