This patch fixes the combination of CONFIG_DISCONTIGMEM and
CONFIG_VIRTUAL_MEM_MAP so that generic kernels will work on all ia64
platforms, including sn2, and also makes sn2 specific kernels work
(which I think is a first).

I've cleaned this patch up heavily based on feedback from yourself,
Christoph and others.  I've tested sn2, zx1, and dig (thanks Xavier!)
specific configurations, as well as a generic configuration that allows
the same binary to boot on zx1, dig, and sn2.