Makefile: Globally enable fall-through warning

Now that all the fall-through warnings have been addressed in the
kernel, enable the fall-through warning globally.

Also, update the deprecated.rst file to include implicit fall-through
as 'deprecated' so people can be pointed to a single location for
justification.

Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: Kees Cook <keescook@chromium.org>
Cc: linux-kbuild@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>