Riven procs life steal on her Q, so while you do massive amount of damage you also get health back. (Mistake, see next two posts)
Frozen mallet is an awful item on Riven. She has so much mobility already that the slow is unnecessary. If you want survivability, get a Randiun's Omen. Gives health, armor, passive attack speed slow and active movement speed slow. Absolutely amazing item.
My personal build:
5 pots 2 wards
First back - brutalizer, if you have enough for bf sword get that; pots
Midgame - you are not rushing BC, you are rushing BT. Early brutalizer is just there to give you even more of a lane dominance. Merc treds eventually.
Then get the BC afterwards. More wards/pots
Late game - GA, but the following are situational. Randuin's Omen, that QSS upgrade (CCs are her enemy), LW, Ravenous Hydra, second BT
Final build could be something like:
Again many items are situational. Adjust accordingly.