Stop setting PG_U in bootstrap mappings.

These mappings are never visible to userspace as they get replaced when
the amd64 pmap is bootstrapped, but there is no need to set PG_U in the
first place.

Reviewed by: alc, kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D24547