kbuild: make distclean work against $(objtree) instead of $(srctree)

This reverts the old commit [1], which seems questionable to me.

It claimed 'make distclean' could not remove editor backup files,
but I believe KBUILD_OUTPUT or O= was set.

When O= is given, Kbuild should always work against $(objtree).
If O= is not given, $(objtree) and $(srctree) are the same, therefore
$(srctree) is cleaned up.

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=dd47df980c02eb33833b2690b033c34fba2fa80d



Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>