[IA64] Turn on CONFIG_HAVE_UNSTABLE_CLOCK

sched_clock() on ia64 is based on ar.itc, so is never
completely synchronized between cpus. On some platforms
(e.g. certain models of SGI Altix) it may be running at
radically different frequencies.

Based on a patch from Dimitri Sivanich which set this
just for SN2 && GENERIC kernels ... it is needed for
all ia64 machines.

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