[Stackless] lib_dirs in setup.py

[Milo]

Hello.

This is for stackless 3.2.1 cloned from Mercurial. In detect_modules()
there are two paths which are currently missing for Debian 7.0:

/lib/x86_64-linux-gnu
/usr/lib/x86_64-linux-gnu

They can be found in /etc/ld.so.conf.d/x86_64-linux-gnu.conf and they
were added because of multiarch support.

I added them to lib_dirs and all the missing "bits" were detected.
Please consider adding them too, thanks!

-- 
Cheers,
Milo