[IA64] enable SGI simulator for generic kernels

Allow the SGI simulator (medusa) to work on generic kernels.  There is
no inherent dependency on an sn2-specific kernel.

Boot tested on Altix, medusa and HP rx2600.

Signed-off-by: Greg Edwards <edwardsg@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>