efi/zboot: arm64: Grab code size from image header

Instead of relying on a dodgy dd hack to copy the image code size from
the uncompressed image's PE header to the end of the compressed image,
let's grab the code size from the text_offset field of the arm64 image
header after decompression, which is where the arm64 specific EFI zboot
make rules will poke the code size when generating the zboot specific
version of the binary Image payload.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>