Reduce warnings, extern "C" cleanup (#20279)

Marlin/src/HAL/STM32/HAL.h

@@ -140,15 +140,19 @@ void _delay_ms(const int delay);
 
 extern "C" char* _sbrk(int incr);
 
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wunused-function"
+#if GCC_VERSION <= 50000
+  #pragma GCC diagnostic push
+  #pragma GCC diagnostic ignored "-Wunused-function"
+#endif
 
 static inline int freeMemory() {
   volatile char top;
   return &top - reinterpret_cast<char*>(_sbrk(0));
 }
 
-#pragma GCC diagnostic pop
+#if GCC_VERSION <= 50000
+  #pragma GCC diagnostic pop
+#endif
 
 //
 // ADC