[flang][runtime] Fix MINVAL([CHARACTER(2)::])

[klausler]

Summary

The result of MINVAL over an empty default character array should
(per the standard) have a 127 in every character position, not just
the first.

[vdonaldson]

nit: The comment assumes MINVAL, although the code is also invoked for MAXVAL.

flang/runtime/extrema.cpp

     if (extremum_) {
       std::memcpy(p, extremum_, byteSize);
     } else {
-      // empty array
-      if constexpr (KIND == 1) { // ASCII
-        *p = IS_MAXVAL ? 0 : 127; // 127 required by standard
-      } else {
-        std::memset(p, IS_MAXVAL ? 0 : 255, byteSize);
-      }
+      // empty array; fill with character 127 if ASCII as required
+      // by the standard, otherwise set all of the bits.
+      std::memset(p, IS_MAXVAL ? 0 : KIND == 1 ? 127 : 255, byteSize);
     }
   }
   bool Accumulate(const Type *x) {