arm: compressed: put zImage header and EFI header in dedicated section

To align the PE/COFF and the ELF headers of the decompressor binary, put
the zImage header and the EFI header in a separate .start section, and
emit it at the start of the Image. This change is necessary for UEFI
based debug tooling to be able to use the vmlinux ELF binary, since it
gets confused if the PE/COFF .text section and the ELF .text section live
at different offsets.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>