efi/zboot: arm64: Grab code size from ELF symbol in payload

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 symbol that is injected into the ELF
object by the Kbuild rules that generate the compressed payload.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>