Define a unified pmap structure for i386.

The overloading of struct pmap for PAE and non-PAE pmaps results in
three distinct layouts for the structure, which is embedded in
struct vmspace. This causes a large number of duplicate structure
definitions in the i386 kernel's CTF type graph.

Since most pmap fields are the same in the two pmaps, simply provide
side-by-side variants of the fields that are distinct, using fixed-size
types.

PR: 242689
Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22896