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.

sim/lm32/local.mk

@@ -35,6 +35,7 @@ BUILT_SOURCES += %D%/eng.h
 
 ## This makes sure build tools are available before building the arch-subdirs.
 SIM_ALL_RECURSIVE_DEPS += $(%C%_BUILD_OUTPUTS)
+%D%/modules.c: | $(%C%_BUILD_OUTPUTS)
 
 ## FIXME: Use of `mono' is wip.
 %D%/mloop.c %D%/eng.h: %D%/stamp-mloop ; @true