Alright. I'm a Morde main, but I've played Riven enough times to know how she works.
She can get away with building all AD, but cleaver IS good on her, for CDR, hp cuz shes melee, and armor shred that eventually outscales ghostblade. tho I'd have personally rushed Ghostblade for armor pen and the MS active so you are super mobile even after you used your 3 Q's.
Ghostblade>Dusk blade for MS, super high AD, and armor pen (what good is 40 ad and increasing armor shred vs 60 or 70 AD with flat armor pen and constant mobility bonus?) THAN you go Cleaver as a midgame item or second item. Also, if your enemy has lots of HP, get BotRK for Attack Speed, %hp damage, and Life steal from your high AD high pen build.
On Riven, definitely get Lifesteal, at least some. An Armor Pen, lifestealing, higher mobility Riven sounds terrifying.
I can only play melee champs myself, which is why I'm a Morde one trick, but hopefully this helped you some.
Sustain+high damage+high pen= tanks have no meaning and suddenly are very squishy