[IA64] Add "model name" to /proc/cpuinfo

Linux ia64 port tried to decode the processor family number
to something human-readable, but Intel brandnames don't change
synchronously with updates to the family number.  Adopt a more
i386-like approach and just print the family number in decimal.
Add a new field "model name" that uses PAL_BRAND_INFO to find
the official name for the cpu, or on older systems, falls back
to using the well-known codenames (Merced, McKinley, Madison).

Signed-off-by: Tony Luck <tony.luck@intel.com>