Add a new target to clean all *.cmi files. Now it is convenient to use 'make...

Add a new target to clean all *.cmi files. Now it is convenient to use 'make cleanall all stdlib' and then 'make clean_cmi all stdlib'.

generator/Makefile

@@ -44,4 +44,5 @@ clean:
 	rm -rf _build
 	rm -f *.native *.byte
 
-cleanall: clean clean_tests clean_stdlib
+clean_cmi: clean_tests clean_stdlib
+cleanall: clean clean_cmi