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/module/temperature.cpp

@@ -1081,7 +1081,7 @@ void Temperature::init() {
   #endif
 
   // todo: HAL: fix abstraction
-  #ifdef ARDUINO_ARCH_AVR
+  #ifdef __AVR__
     // Use timer0 for temperature measurement
     // Interleave temperature interrupt with millies interrupt
     OCR0B = 128;