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

Marlin/src/HAL/ESP32/HAL.h

@@ -100,10 +100,16 @@ inline void HAL_reboot() {}  // reboot the board or restart the bootloader
 
 void _delay_ms(int delay);
 
-#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
 
 void analogWrite(pin_t pin, int value);