EFI: Make our haiku_loader architecture agnostic

* This is the bulk of the work. Anything else should be
minor cleanups and tweaking.
* riscv64 isn't a viable EFI platform yet.. just acting
as a stand-in to test a non-x86 EFI haiku_loader

Change-Id: Ib03de81e2b562e693987b86d7b4318209fb1c792
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2256
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>