How get assembler/preprocessed code for library
You can get the generated assembly by passing the -S option to
avr-g++. The makefile you are using has a rule for that, so you would
just type

make build-*/my_file.s


to get the assembly for my_file.cpp.

For the pre-processed C++, you use the -E option. There is no rule for
that in the makefile, so you will have to write yo own, or manually
invoke the compiler. At a minimum:

avr-g++ -mmcu=my_mcu -E my_file.cpp &gt; my_file.i


You may want to add -DF_CPU=... -DARDUINO=... just like the makefile
does when compiling the same file.