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/HAL_AVR.cpp

@@ -23,11 +23,11 @@
 /**
  * Description: HAL for AVR
  *
- * For ARDUINO_ARCH_AVR
+ * For __AVR__
  */
 
 
-#ifdef ARDUINO_ARCH_AVR
+#ifdef __AVR__
 
 // --------------------------------------------------------------------------
 // Includes

Marlin/src/HAL/HAL_AVR/HAL_AVR.h

@@ -22,7 +22,7 @@
 /**
  * Description: HAL for AVR
  *
- * For ARDUINO_ARCH_AVR
+ * For __AVR__
  */
 
 

Marlin/src/HAL/HAL_AVR/HAL_spi_AVR.cpp

@@ -28,10 +28,10 @@
 /**
  * Description: HAL for AVR - SPI functions
  *
- * For ARDUINO_ARCH_AVR
+ * For __AVR__
  */
 
-#ifdef ARDUINO_ARCH_AVR
+#ifdef __AVR__
 
 // --------------------------------------------------------------------------
 // Includes
@@ -210,4 +210,4 @@ void spiBegin (void) {
 #endif  // SOFTWARE_SPI
 
 
-#endif // ARDUINO_ARCH_AVR
+#endif // __AVR__

Marlin/src/HAL/HAL_AVR/MarlinSerial.cpp

@@ -28,7 +28,7 @@
  * Modified 28 September 2010 by Mark Sproul
  * Modified 14 February 2016 by Andreas Hardtung (added tx buffer)
  */
-#ifdef ARDUINO_ARCH_AVR
+#ifdef __AVR__
 
 #include "MarlinSerial.h"
 #include "../../Marlin.h"

Marlin/src/HAL/HAL_AVR/persistent_store_impl.cpp

@@ -1,4 +1,4 @@
-#ifdef ARDUINO_ARCH_AVR
+#ifdef __AVR__
 
 #include "../persistent_store_api.h"
 
@@ -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__

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__

Marlin/src/HAL/HAL_AVR/watchdog_AVR.cpp

@@ -20,7 +20,7 @@
  *
  */
 
-#ifdef ARDUINO_ARCH_AVR
+#ifdef __AVR__
 
 #include "../../inc/MarlinConfig.h"
 
@@ -58,4 +58,4 @@ void watchdog_init() {
 #endif // WATCHDOG_RESET_MANUAL
 
 #endif // USE_WATCHDOG
-#endif // ARDUINO_ARCH_AVR
+#endif // __AVR__