core: Define F_CPU for ARM targets

Add F_CPU definition for retrive CPU frequency.

Co-authored-by: Martino Facchin <m.facchin@arduino.cc>
Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>

Author: soburi

cores/arduino/Arduino.h

@@ -144,6 +144,10 @@ int digitalPinToPinIndex(pin_size_t pinNumber);
 void analogWriteResolution(int bits);
 #endif
 
+#if defined(__arm__)
+#define F_CPU (SystemCoreClock)
+#endif
+
 #include <variant.h>
 
 #if !defined(LED_BUILTIN) && defined(ZARD_LED_BUILTIN)