Revert "core: make the jailhouse-header arch-independent"

This reverts commit 17f8453876f8b143022278e6438c8ebb157d1821.

It turned out to break 32-bit architectures: At least core_size and
entry must be of the size the hypervisor uses for pointer, otherwise
link-time initialization of those fields fail.

We will need a more sophisticated solution if including the header in
i386 mode is really necessary.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>