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_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__