2 very high elo player guides:
http://www.solomid.net/guides.php?g=8379
http://www.solomid.net/guides.php?g=16914
Bloodthirster > IE
AD > pure defense
All of you say Brutalizer, I disagree. Youmuu's is useless on Riven because you don't need AS since you're just combo'ing attacks with an AA in between. Sure cdr/arp are good, but you are better off with something else. You can buy brutalizer as a luxury item, but replace it later, don't upgrade it.
9/10 games I'll go boots (usually tenacity) -> BF -> BT -> GA -> enemy team dependant, but probably last whisper. If I get a hard lane or a annoying jungler I might go Wriggles first, then BT. Since you get GA, you just want to wreck as much havoc as you can (E giving you a large shield) on their carries, die (while your team mashes on) and if necessary W -> E -> Q Q Q to safety.