it counts are the cards in your original deck being of the same cost, not added later so you can play the new cthun in an even cost deck and prince malchezaar (add 5 random legendaries at the start of the duel in your deck) in an odd deck
I'm pretty sure hysteria tested this in one of his myth buster videos and it depends on which order the effects trigger. Let's say you play c'thun in an even deck. If genn's effect triggers first you get 1 mana hero power then c'thun's effect triggers adding the spells to your deck. If it is the other way around then c'thun adds the spells to your deck and genn's effect doesn't trigger. In the video it is said that the first minion between these 2 you get to draw will trigger the effect first, so if you'd draw genn before c'thun (before start of game effect) you would have both effects active.