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

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.

Differential Revision: https://reviews.llvm.org/D155976

flang/runtime/extrema.cpp

@@ -800,7 +800,7 @@ template <int KIND> class Norm2Accumulator {
   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};