Merge the md_page structs for AIM and Book-E into a single unioned struct

Summary:
Ports like sysutils/lsof troll through kernel structures, and
therefore include kernel headers and all the dirty secrets involved. struct
vm_page includes the struct md_page inline, which currently is only defined
if AIM or BOOKE is defined. Thus, by default, sysutils/lsof cannot build,
due to the struct md_page having an incomplete type. Fix this by merging
the two struct definitions into an anonymous struct-union.

A similar change could be made to unify the pmap structures as well.

Reviewed By: nwhitehorn
Differential Revision: https://reviews.freebsd.org/D16232