golang.org/x/tools/go/analysis: pass "gccgo" to types.SizesFor

golang.org/x/tools/go/analysis: pass "gccgo" to types.SizesFor

    For the gofrontend copy, change calls to types.SizesFor to pass
    "gccgo" rather than "gc".  Leave the asmdecl pass unchanged since that
    pass is gc-specific anyhow.

    This has been fixed in a better way in the external repo by
    https://golang.org/cl/158317 and friends, but that is not in 1.12, so
    use this approach for now.

    Reviewed-on: https://go-review.googlesource.com/c/158842

From-SVN: r268153