Function ref_dot

tests/sycl/math.cpp:164–170

  template<typename DT, int D>
  DT ref_dot(vec<DT, D> a, vec<DT, D> b) {
    DT ret = DT{0};
    for(int c = 0; c < D; ++c) {
      ret += comp(a, c) * comp(b, c);
    }
    return ret;
  }
  double ref_dot(double a, double b) {
    return a * b;
  }