bump(sys-devel/gcc): sync with upstream

This updates our compiler to gcc-4.7.3-r1 which has been stable in
Gentoo for a long time now. One interesting thing in this compiler is it
will report errors if you attempt to link against system libraries when
building in an alternate sysroot like /build/amd64-usr which should help
catch a variety of mistakes.
Merge pull request #133 from marineam/gcc

bump(sys-devel/gcc): sync with upstream