Fix PAE modules build on i386.

Reimplement PAE version of pte_load() by copying/pasting the
atomic_load_acq_64_i586() into it definition. pmap_kextract() is defined
as inline and uses pte_load() in its body, so the pte_load() should be
available when pmap.h is included. On stable/11, the atomic inlines are
not exposed to modules.

This is a direct commit to stable/11.

Reported by: dim
Sponsored by: The FreeBSD Foundation