compiler: use more accurate array type in Gogo::register_gc_vars

compiler: use more accurate array type in Gogo::register_gc_vars

    The length of the roots array in Gogo::register_gc_vars was being
    computed incorrectly, potentially leading to type clashes in the back
    end. Compute an accurate length for this array ahead of time.

    Reviewed-on: https://go-review.googlesource.com/34624

From-SVN: r243974