Create `cpp/avr/` or `cpp/avr_includes/avr` as appropriate to support <avr/*.h>

[ianfixes]

Issue Summary

Compilation for AVR stuff will fail due to lack of file existence in cpp/arduino/avr/*. Needs to be fixed.

https://github.com/ianfixes/arduino_ci/pull/115#issuecomment-466848042

To preserve separation of the avr/ hack and actual mocks, we will need to add a new cpp include directory alongside arduino.