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/delta.cpp

@@ -88,7 +88,7 @@ void recalc_delta_settings(float radius, float diagonal_rod) {
  *   (see above)
  */
 
-#if ENABLED(DELTA_FAST_SQRT) && defined(ARDUINO_ARCH_AVR)
+#if ENABLED(DELTA_FAST_SQRT) && defined(__AVR__)
   /**
    * Fast inverse sqrt from Quake III Arena
    * See: https://en.wikipedia.org/wiki/Fast_inverse_square_root

Marlin/src/module/delta.h

@@ -63,7 +63,7 @@ void recalc_delta_settings(float radius, float diagonal_rod);
  *   (see above)
  */
 
-#if ENABLED(DELTA_FAST_SQRT) && defined(ARDUINO_ARCH_AVR)
+#if ENABLED(DELTA_FAST_SQRT) && defined(__AVR__)
   /**
    * Fast inverse sqrt from Quake III Arena
    * See: https://en.wikipedia.org/wiki/Fast_inverse_square_root

Marlin/src/module/stepper.cpp

@@ -46,7 +46,7 @@
 
 #include "stepper.h"
 
-#ifdef ARDUINO_ARCH_AVR
+#ifdef __AVR__
   #include "speed_lookuptable.h"
 #endif
 
@@ -1139,7 +1139,7 @@ void Stepper::init() {
     E_AXIS_INIT(4);
   #endif
 
-  #ifdef ARDUINO_ARCH_AVR
+  #ifdef __AVR__
     // waveform generation = 0100 = CTC
     SET_WGM(1, CTC_OCRnA);
 

Marlin/src/module/stepper.h

@@ -45,7 +45,7 @@
 
 #include "stepper_indirection.h"
 
-#ifdef ARDUINO_ARCH_AVR
+#ifdef __AVR__
   #include "speed_lookuptable.h"
 #endif
 

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;