x86: Relocate Multiboot structures where we know they will be

x86: Relocate Multiboot structures where we know they will be
accessible. GRUB2 seems to like to stick them really high sometimes
(just below 4GB).

The 32-bit C code framework that this sets up can also be used for
other stuff in future:
 * early cmdline parsing
 * relocating multiboot modules so they too are guaranteed accessible

Its interaction with normal Xen start-of-day, and with the 16-bit
assembly trampoline, needs a bit of thought.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>