sysutils/edk2: add Xen platform

As part of this make the gcc aarch64 dependency conditional to
building a firmware for that architecture, and make the nasm package a
build dependency for the X64 (amd64) targets.

Sponsored by: Citrix Systems R&D
Reviewed by: manu
Approved by: manu
Differential revision: https://reviews.freebsd.org/D30225