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

Marlin/src/HAL/LINUX/HAL.h

@@ -79,10 +79,16 @@ extern HalSerial usb_serial;
 inline void HAL_init() {}
 
 // Utility functions
-#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
 
 // ADC
 #define HAL_ADC_VREF           5.0