UPSTREAM: 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: dc5723b0 ("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


(cherry picked from commit 4c7858b9)
Bug: 187129171
Signed-off-by: Connor O'Brien <connoro@google.com>
Change-Id: I2126e71b9cf7d70e7192ac267f6b7d8fdf985bd3