Add rules for tracking generated subdir modules.c files. This doesn't
actually generate the file from the top-level, but allows us to add
rules that need to be ordered wrt it. Once those changes land, we can
rework this to actually generate from the top-level.
This currently builds off of the objects that go into the libsim.a as
we don't build those from the top-level either. Once we migrate that
up, we can switch this to the source files directly. It's a bit hacky
overall, but makes it easier to migrate things in smaller chunks, and
we aren't going to keep this logic long term.
437eeee95c