HardwareSerial: Define no-op serialEvent() functions for clang

GCC automatically resolves weakly defined empty symbols to null if not overridden, but Clang doesn't resolve it this way. This causes undefined reference errors during linking.

To address this, define empty no-op serialEvent() functions that can be overridden later since its weakly defined, ensuring compatibility with Clang and fixing the build issue.
ref: avr-llvm/clang#9

Signed-off-by: Dakkshesh <dakkshesh5@gmail.com>

Author: beakthoven

cores/arduino/HardwareSerial.cpp

@@ -44,21 +44,33 @@
 #if defined(HAVE_HWSERIAL0)
   void serialEvent() __attribute__((weak));
   bool Serial0_available() __attribute__((weak));
+
+  void serialEvent() { }
+  bool Serial0_available() { return false; }
 #endif
 
 #if defined(HAVE_HWSERIAL1)
   void serialEvent1() __attribute__((weak));
   bool Serial1_available() __attribute__((weak));
+
+  void serialEvent1() { }
+  bool Serial1_available() { return false; }
 #endif
 
 #if defined(HAVE_HWSERIAL2)
   void serialEvent2() __attribute__((weak));
   bool Serial2_available() __attribute__((weak));
+
+  void serialEvent2() { }
+  bool Serial2_available() { return false; }
 #endif
 
 #if defined(HAVE_HWSERIAL3)
   void serialEvent3() __attribute__((weak));
   bool Serial3_available() __attribute__((weak));
+
+  void serialEvent3() { }
+  bool Serial3_available() { return false; }
 #endif
 
 void serialEventRun(void)