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

Marlin/src/HAL/SAMD51/HAL.h

@@ -135,10 +135,16 @@ void HAL_idletask();
 //
 FORCE_INLINE void _delay_ms(const int delay_ms) { delay(delay_ms); }
 
-#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
+
 int freeMemory();
-#pragma GCC diagnostic pop
+
+#if GCC_VERSION <= 50000
+  #pragma GCC diagnostic pop
+#endif
 
 #ifdef __cplusplus
   extern "C" {