sgi-xp: enable building of XPC/XPNET on x86_64

Get XPC/XPNET to build on x86_64.  Trying to modprobe them up on a non-UV
or sn2 system will result in a -ENODEV.

Signed-off-by: Dean Nelson <dcn@sgi.com>
Cc: Jack Steiner <steiner@sgi.com>
Cc: "Luck, Tony" <tony.luck@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>