Linux TBB should better detect wrong 32-bit vs 64-bit choice

I ran the 686 TBB (1.1.14-dev) on my 64-bit wheezy/sid, and got

arma@last-request:/tmp/tor-browser_en-US$ ./start-tor-browser 

Launching Tor Browser Bundle for Linux in /tmp/tor-browser_en-US
./App/vidalia: error while loading shared libraries: libXext.so.6: cannot open shared object file: No such file or directory
Vidalia exited abnormally.  Exit code: 127

It seems we should do some sort of detection if somebody has picked the wrong arch, and tell them that rather than a cryptic lib failure?

(A short-term bandaid might be to add a "Are you sure you're not on a 64-bit platform?" hint when Vidalia exits abnormally.)

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