stand/efi: add modulep to kernel metadata

This mirrors the functionality of the BIOS amd64 bi_load function,
that stashes the absolute address of the module metadata. This is
required for booting as a Xen dom0 that does relocate the modulep and
the loaded modules, and thus requires adjusting the offset.

No functional change introduced, further patches will make use of this
functionality for Xen dom0 loading.

Sponsored by: Citrix Systems R&D
Reviewed by: imp
Differential revision: https://reviews.freebsd.org/D28496