[IA64] Implement futex primitives

Implement futex_atomic_op_inuser() and futex_atomic_cmpxchg_inatomic()
on IA64 in order to fully support all futex functionality.

Signed-off-by: Jakub Jelinek <jakub@redhat.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>