mboot64: handing off a memory to booted kernel.

Currently support pseudo-Multiboot ELF kernels, although all
this logic needs to be abstracted out.

Also use proper unaligned accesses. MMU is off, and thus
all accesses are Device-nGnRnE and thus would trigger
alignment traps.

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>