Fix incorrect reading of 32-bit modinfo by 64-bit loaders.

The various structures in the mod_metadata set of a FreeBSD kernel and
modules contain pointers. The FreeBSD loader correctly deals with a
mismatch in loader and kernel pointer size (e.g. 32-bit i386/ppc
loader, loading 64-bit amd64/ppc64 kernels), but wasn't dealing with
the inverse case where a 64-bit loader was loading a 32-bit kernel.

Reported by: ktcallbox@gmail.com with a bhyve/i386 and ZFS root install
Differential Revision: https://reviews.freebsd.org/D1129
Reviewed by: neel, jhb
MFC after: 1 week