kbuild: remove tag files by distclean instead of mrproper

It takes somewhat long time to generate these tag files.
Keep such precious files until we run 'make distclean'.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1378,11 +1378,15 @@ CLEAN_FILES += modules.builtin.modinfo
 MRPROPER_DIRS  += include/config include/generated          \
 		  arch/$(SRCARCH)/include/generated .tmp_objdiff
 MRPROPER_FILES += .config .config.old .version \
-		  Module.symvers tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS \
+		  Module.symvers \
 		  signing_key.pem signing_key.priv signing_key.x509	\
 		  x509.genkey extra_certificates signing_key.x509.keyid	\
 		  signing_key.x509.signer vmlinux-gdb.py
 
+# Directories & files removed with 'make distclean'
+DISTCLEAN_DIRS  +=
+DISTCLEAN_FILES += tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS
+
 # clean - Delete most, but leave enough to build external modules
 #
 clean: rm-dirs  := $(CLEAN_DIRS)
@@ -1415,9 +1419,14 @@ mrproper: clean $(mrproper-dirs)
 
 # distclean
 #
+distclean: rm-dirs  := $(wildcard $(DISTCLEAN_DIRS))
+distclean: rm-files := $(wildcard $(DISTCLEAN_FILES))
+
 PHONY += distclean
 
 distclean: mrproper
+	$(call cmd,rmdirs)
+	$(call cmd,rmfiles)
 	@find $(srctree) $(RCS_FIND_IGNORE) \
 		\( -name '*.orig' -o -name '*.rej' -o -name '*~' \
 		-o -name '*.bak' -o -name '#*#' -o -name '*%' \