kbuild: make clean rule robust against too long argument error

Commit cd968b9 ("kbuild: make built-in.a rule robust against too
long argument error") made a build rule robust against "Argument list
too long" error.

Eugeniu Rosca reported the same error occurred when cleaning an external
module.

The $(obj)/ prefix can be a very long path for external modules.

Apply a similar solution to 'make clean'.

Reported-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Tested-by: Eugeniu Rosca <erosca@de.adit-jv.com>

scripts/Makefile.clean

@@ -37,8 +37,10 @@ __clean-files   := $(wildcard $(addprefix $(obj)/, $(__clean-files)))
 
 # ==========================================================================
 
+# To make this rule robust against "Argument list too long" error,
+# remove $(obj)/ prefix, and restore it by a shell command.
 quiet_cmd_clean = CLEAN   $(obj)
-      cmd_clean = rm -rf $(__clean-files)
+      cmd_clean = printf '$(obj)/%s ' $(patsubst $(obj)/%,%,$(__clean-files)) | xargs rm -rf
 
 __clean: $(subdir-ymn)
 ifneq ($(strip $(__clean-files)),)