[IA64] fix siblings field value in /proc/cpuinfo

Fix the "siblings" field value in /proc/cpuinfo so that it now shows the
number of siblings as seen by OS, instead of what is available from
hardware perspective.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>