gotest: fix handling of --goarch option

gotest: fix handling of --goarch option

    I managed to add a new --goarch option to gotest without noticing that
    the script already had one.  Worse, they set different variables.
    Remove the old option in favor of the new one.

    Reviewed-on: https://go-review.googlesource.com/16613

From-SVN: r229734