sim: modules: trigger generation from top-level

sim: modules: trigger generation from top-level

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.