stand/multiboot2: add support for booting a Xen dom0 in UEFI mode

Add some basic multiboot2 infrastructure to the EFI loader in order to
be capable of booting a FreeBSD/Xen dom0 when booted from UEFI.

Only a very limited subset of the multiboot2 protocol is implemented
in order to support enough to boot into Xen, the implementation
doesn't intend to be a full multiboot2 capable implementation.

Such multiboot2 functionality is hooked up into the amd64 EFI loader,
which is the only architecture that supports Xen dom0 on FreeBSD.

The options to boot a FreeBSD/Xen dom0 system are exactly the same as
on BIOS, and requires setting the xen_kernel and xen_cmdline options
in loader.conf.

Sponsored by: Citrix Systems R&D
Reviewed by: tsoome, imp
Differential revision: https://reviews.freebsd.org/D28497