Add support to allocate more space before and after a current allocation

[andrew]

Summary

With a very large kernel or module the staging area may be too small to
hold it. When this is the case try to allocate more space before failing
in the efi copyin/copyout/readin functions.

Test Plan

Loaded a 69M kernel with a 32M staging area on arm64.

[imp]

Not seeing any holes here

[tsoome]

Seems ok.
Note, the issue about the staging size is not really if we can or can not allocate specific size, but if we have enough target space to copy it. I have starting to port other change (https://reviews.freebsd.org/D22309), the idea there is that we should determine if the target location has enough space. The bios version is simple as there we own the memory and we can not load directly to target address, in UEFI case we need to allocate, load and after switching off the BS, only then we can copy.