go/types: implement SizesFor for gccgo

go/types: implement SizesFor for gccgo

    Move the architecture-specific settings out of configure.ac into a new
    shell script goarch.sh.  Use the new script to collect the values for
    all architectures to make them available in go/types.

    Also fix cmd/vet to pass the right compiler when it calls SizesFor.

    This fixes cmd/vet for systems that are not implemented in the gc
    toolchain, such as alpha and ia64.

    Reviewed-on: https://go-review.googlesource.com/87635

From-SVN: r256655