Use reinterpret_cast to cast __FlashStringHelper to const char*

The cast from __FlashStringHelper to const char* produced the following warning:
```
/Users/Lauszus/GitHub/Arduino/build/macosx/work/Arduino.app/Contents/Resources/Java/hardware/arduino/avr/cores/arduino/Print.cpp: In member function 'size_t Print::print(const __FlashStringHelper*)':
/Users/Lauszus/GitHub/Arduino/build/macosx/work/Arduino.app/Contents/Resources/Java/hardware/arduino/avr/cores/arduino/Print.cpp:44: warning: '__progmem__' attribute ignored
```

As the F() macro is defined this way: https://github.com/arduino/Arduino/blob/c86eed942dd3d7d0bced4fb26fb28ee57fccff07/hardware/arduino/avr/cores/arduino/WString.h#L38. We should use reinterpret_cast to cast it back to a const char*.