How do you counter build Riven?

Well if armor and health aren't working I guess your only option is Magic Resistance then

Armor and HP do work, but not as a last-slot effort. You need to grab it early enough to make a significant dent in her damage output.

Armor and HP work in the early game, to prevent her from snowballing. If you have better scaling (which you should), you just need to get to that point. Make sure to ward up and keep Riven from roaming and getting fed from elsewhere. If you can deny her kills, and hopefully deny her farm as well, eventually there will be a point she becomes useless.

The point of armor/hp against Riven is doing it early, when she begins to snowball. You deny her that chance, you get her behind while you scale better, and then you get your items and wreck her face.

If all else fails, you could always ask the support and jungler for help. Nobody wants a fed Riven on the enemy team.

it's not like she does true damage