efi/arm64: libstub: Split off kernel image relocation for builtin stub

The arm64 build of the EFI stub is part of the core kernel image, and
therefore accesses section markers directly when it needs to figure out
the size of the various section.

The zboot decompressor does not have access to those symbols, but
doesn't really need that either. So let's move handle_kernel_image()
into a separate file (or rather, move everything else into a separate
file) so that the zboot build does not pull in unused code that links to
symbols that it does not define.

While at it, introduce a helper routine that the generic zboot loader
will need to invoke after decompressing the image but before invoking
it, to ensure that the I-side view of memory is consistent.

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