[AVR] SPI atomicity fix avr (part 1/2 of #2376)

This pull request broke into 6 simpler commits the big patch in #2376.

/cc @xxxajk @PaulStoffregen 

IMHO in SPI.cpp the include is not needed at all because:

- SPI.cpp includes `"SPI.h"`
- SPI.h includes `<Arduino.h>`
- Arduino.h includes `"pins_arduino.h"`

BTW I agree with you that this line:
https://github.com/arduino/Arduino/blob/master/hardware/arduino/cores/arduino/Arduino.h#L237
should be:
```
#include <pins_arduino.h>
```