i386 pmap: Adapt recent amd64/arm64 superpage improvements

Don't recompute mpte during promotion.

Optimize MADV_WILLNEED on existing superpages.

Standardize promotion conditions across amd64, arm64, and i386.

Stop requiring the accessed bit for superpage promotion.

Tidy up pmap_promote_pde() calls.

Retire PMAP_INLINE. It's no longer used.

Note: Some of these changes are a prerequisite to fixing a panic that
arises when attempting to create a wired superpage mapping by
pmap_enter(psind=1) (as opposed to promotion).

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D41944