Also remove .rustc_info.json in clean-mmtk target

Author: peterzhu2118

gc/mmtk/extconf.rb

@@ -37,7 +37,8 @@
 
   .PHONY: clean-mmtk
   clean-mmtk:
-		-$(Q)$(RM_RF) debug release
+  	-$(Q)$(RM_RF) debug release
+  	-$(Q)$(RM) .rustc_info.json
 MAKEFILE
 
 File.open("Makefile", "w") { |file| file.puts(makefile) }