Assembler files in sketches (presumably) won't work

[matthijskooijman]

From reviewing the code (in response to a question on IRC) I believe that assembler files .S in sketches will not currently work. It seems there is code to support .S files using recipes, but .S files are not listed as files to copy into the build dir here: https://github.com/arduino/arduino-builder/blob/master/src/arduino.cc/builder/builder.go#L44-L45

I haven't actually tested this, but I believe things are broken and a fix should be a matter of adding .S files to that list.

At the same time, it might be useful to also add .inc files, since it seems those are sometimes used as assembly include files.

For some example assembler files to use for testing, see https://github.com/digistump/DigisparkArduinoIntegration/tree/master/libraries/DigisparkKeyboard