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

## 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`.
