[flang] build fix (#121032)

Place floating point environment calls under '#ifdef __USE_GNU'.

Author: vdonaldson

flang/runtime/exceptions.cpp

@@ -84,7 +84,7 @@ uint32_t RTNAME(MapException)(uint32_t excepts) {
 // Check if the processor has the ability to control whether to halt or
 // continue execution when a given exception is raised.
 bool RTNAME(SupportHalting)([[maybe_unused]] uint32_t except) {
-#if (defined(__arm__) || defined(__aarch64__)) && !defined(_WIN32)
+#ifdef __USE_GNU
   except = RTNAME(MapException)(except);
   int currentSet = fegetexcept(), flipSet, ok;
   if (currentSet & except) {