efi: Refactor our EFI code to use fuchsia's cleaner EFI headers.

* Drop gnu-efi

Change-Id: Ib601fc8ced49b18281b6b98cf861a5aef1b9c065
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2026
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>