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_AVR/servo_AVR.cpp

@@ -51,7 +51,7 @@
  *
  */
 
-#ifdef ARDUINO_ARCH_AVR
+#ifdef __AVR__
 
 #include "../../inc/MarlinConfig.h"
 
@@ -215,4 +215,4 @@ void finISR(timer16_Sequence_t timer) {
 
 #endif // HAS_SERVOS
 
-#endif // ARDUINO_ARCH_AVR
+#endif // __AVR__