What's the AP of Eldrad's Staff and Yreil's spear?
You are correct in that neither weapon entitles a model wounded by them to a standard armour save. Invulnerable armour saves would be taken as normal.

The approach to determining this is if a weapon or a weapon's special rules are not covered in the Rule Book ("Big Yellow Book" or BYB, 6th edition) then you take the description as written in the Codex.

In the case of these two weapons:

Staff of Ulthamar

There is no mention that this is a weapon listed in the BRB, and so we use the description on p51 of the current Codex: Eldar:


  "In combat it always wounds on a roll of 2+ and ignores armour saves."


The Spear of Twilight

The Codex mentions on page 53 that this is a Singing Spear and adds a further rule, ignoring armour saves, to that weapon description:


  "It is a Singing Spear that ignores armour saves (see page 27)."


Referring to Page 27, the Singing Spear is a weapon that is similar to a Witchblade but note that it is not listed as a Witchblade. It has it's own weapon profile, rules and description listed in this Codex and these are not overridden by anything in the new rule book. 

As a result we use the profile detailed on this page, remembering to add in the "ignores armour saves" rules mentioned in the Spear of Twilight's own description.