[flang][runtime] Fix NORM2([negative, ...])

[klausler]

Summary

NORM2 is broken for arrays that start with a negative number
because it sets the initial running max_ value to that number
rather than to its absolute value.

[vzakhari]

Thank you!

flang/runtime/extrema.cpp

#endif
   void Reinitialize() { max_ = sum_ = 0; }
   template <typename A> void GetResult(A *p, int /*zeroBasedDim*/ = -1) const {
     // m * sqrt(1 + sum((others(:)/m)**2))
     *p = static_cast<Type>(max_ * std::sqrt(1 + sum_));
   }
   bool Accumulate(Type x) {
     auto absX{std::abs(static_cast<AccumType>(x))};
     if (!max_) {
-      max_ = x;
+      max_ = absX;
     } else if (absX > max_) {
       auto t{max_ / absX}; // < 1.0
       auto tsq{t * t};
       sum_ *= tsq; // scale sum to reflect change to the max
       sum_ += tsq; // include a term for the previous max
       max_ = absX;
     } else { // absX <= max_
       auto t{absX / max_};