i386: Add a leaf PTP when pmap_enter(psind=1) creates a wired mapping

Let pmap_enter_pde() create wired mappings. In particular, allocate a
leaf PTP for use during demotion. This is a step towards reverting
commit 64087fd7f372.

Reviewed by: alc, kib, markj
Sponsored by: Google, Inc. (GSoC 2023)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D41635