kbuild: Move .thinlto-cache removal to 'make clean'

Instead of 'make distclean', 'make clean' should remove build artifacts
unneeded by external module builds. Obviously, you do not need to keep
this directory.

Fixes: dc5723b02e ("kbuild: add support for Clang LTO")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210225193912.3303604-1-masahiroy@kernel.org

Makefile

@@ -1491,7 +1491,7 @@ endif # CONFIG_MODULES
 # Directories & files removed with 'make clean'
 CLEAN_FILES += include/ksym vmlinux.symvers \
 	       modules.builtin modules.builtin.modinfo modules.nsdeps \
-	       compile_commands.json
+	       compile_commands.json .thinlto-cache
 
 # Directories & files removed with 'make mrproper'
 MRPROPER_FILES += include/config include/generated          \
@@ -1505,7 +1505,7 @@ MRPROPER_FILES += include/config include/generated          \
 		  *.spec
 
 # Directories & files removed with 'make distclean'
-DISTCLEAN_FILES += tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS .thinlto-cache
+DISTCLEAN_FILES += tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS
 
 # clean - Delete most, but leave enough to build external modules
 #