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

Marlin/src/HAL/TEENSY35_36/HAL.h

@@ -103,12 +103,16 @@ inline void HAL_reboot() {}  // reboot the board or restart the bootloader
 
 FORCE_INLINE void _delay_ms(const int delay_ms) { delay(delay_ms); }
 
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wunused-function"
-extern "C" {
-  int freeMemory();
-}
-#pragma GCC diagnostic pop
+#if GCC_VERSION <= 50000
+  #pragma GCC diagnostic push
+  #pragma GCC diagnostic ignored "-Wunused-function"
+#endif
+
+extern "C" int freeMemory();
+
+#if GCC_VERSION <= 50000
+  #pragma GCC diagnostic pop
+#endif
 
 // ADC