Fix building .i / .s files for testing

E.g. make arch/i386/kernel/process.[is] used to work at least
in the cases where the object wouldn't need any additional command
line flags defined in the local subdir.

Get back this behavior, I think I know how to do this correctly, too,
but that's for later.

Rules.make

@@ -378,7 +378,7 @@ endif
 # function to only execute the passed command if necessary
 
 if_changed = $(if $(strip $? \
-		          $(filter-out $($(1)),$(cmd_$@))\
-			  $(filter-out $(cmd_$@),$($(1)))),\
-	       @echo $($(1)); $($(1)); echo 'cmd_$@ := $($(1))' > .$@.cmd)
+		          $(filter-out $($(1)),$(cmd_$(@F)))\
+			  $(filter-out $(cmd_$(@F)),$($(1)))),\
+	       @echo $($(1)); $($(1)); echo 'cmd_$(@F) := $($(1))' > $(@D)/.$(@F).cmd)