Improve AVR arch detection

Replace ARDUINO_ARCH_AVR with __AVR__ to better detect architecture for non-Arduino dev environments.  Resolves compile failure in PIO for 8-bit Teensduino targets

More info:
https://forum.pjrc.com/threads/33234-Using-Teensyduino-Selecting-Teensy-3-2-3-1-board-has-incorrect-platform-define
http://www.atmel.com/webdoc/avrlibcreferencemanual/using_tools_1using_avr_gcc_mach_opt.html

Marlin/src/HAL/HAL_DUE/persistent_store_impl.cpp

@@ -52,4 +52,4 @@ void read_data(int &pos, uint8_t* value, uint16_t size, uint16_t *crc) {
 }
 
 #endif // EEPROM_SETTINGS
-#endif // ARDUINO_ARCH_AVR
+#endif // __AVR__