Cabal builder: enable SSE2 on i686

Since the hashable package is already SSE2 only, this doesn't harm us.
But it gives us free performance gains and test success for some
numerical libraries.  This only matters on i686, see
http://www.haskell.org/ghc/docs/7.6.3/html/users_guide/bugs.html.

This commit is extra careful to only trigger a full cabal rebuild for
the i686 platform and not for others.