loader.efi: to preserve heap space, use AllocatePages() for shadow_fb

shadow FB size could be rather large and depends on resolution,
instead of using heap, allocate dedicated space outside of heap.

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D33600

(cherry-picked from 221376db0cb77035e325baf8b54d6224925b41f8)