Only include avr/io.h if __AVR__ is defined

When this file is included, but no MCU type (e.g. `__AVR_ATmega328p__`)
is defined, a warning is generated and only a partial set of
AVR-specific macros is defined.

When using a non-AVR target, no warning should be generated and none of
these AVR-specific macros shoudl be made available, so this only
includes avr/io.h from `Godmode.h` when `__AVR__` is defined, and do not
include it from `avr/pgmspace.h` at all, since it is not actually
needed there.

This introduces a small compatibility issue: Until recently, `__AVR__` was
never defined when running unittests. All included boards now do so, but
if anyone has defined custom AVR boards without defining `__AVR__` in
their `.arduino-ci.yaml` file, those would no longer work. This is easy
enough to fix, though, just add the `__AVR__` define.

Author: matthijskooijman

cpp/arduino/Godmode.h

@@ -1,6 +1,8 @@
 #pragma once
 #include "ArduinoDefines.h"
+#if defined(__AVR__)
 #include <avr/io.h>
+#endif
 #include "WString.h"
 #include "PinHistory.h"
 

cpp/arduino/avr/pgmspace.h

@@ -14,7 +14,6 @@ out = externs.map {|l| l.split("(")[0].split(" ")[-1].gsub("*", "") }.uniq
 out.each { |l| puts d(l) }
 */
 
-#include <avr/io.h>
 #include <string.h>
 
 #define PROGMEM