[IA64] use snprintf() on features field of /proc/cpuinfo

Some patches have turned up on xen-devel recently to convert strcpy()
to safer alternatives and so forth.  While reviewing those patches
I noticed that the features string building could be cleaned up.

This patch uses snprintf() instead of strcpy() and direct character
pointer manipulation.  It makes the features string building safe and
gets rid of the special case for features output in show_cpuinfo()

Additionally I removed the (int) cast of ARRAY_SIZE, which seems to
serve no purpose.

Signed-off-by: Aron Griffis <aron@hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>