Build RPMs for i686 rather than i386

The default target (i386-linux) causes flags like "-march i386" to be
added, which breaks on recent Fedora releases (18 and up), resulting
in errors like:

  /usr/lib/gcc/i686-redhat-linux/4.7.2/../../../../include/c++/4.7.2/ext/atomicity.h:48: undefined reference to `__atomic_fetch_add_4'

So set the target to i686-linux.

http://hydra.nixos.org/build/6567357