Cross amd64 Linux GCC 4.7.2 -> GCC HEAD (20130208) Windows multilib. mingw-w64 HEAD.
Long and painful. Not including Gentoo-specific patches except the actual ebuild. One has to patch eclasses and crossdev too if wants to use OS infrastructure.
Including only relevant and incomplete patch for mingw-w64. Decided against including all the GNU cruft fixes or forking it.