Makefile: Enable -Wzero-length-bounds

With all known internal zero-length accesses fixed, it is possible to
enable -Wzero-length-bounds globally. Since this is included by default
in -Warray-bounds, we just need to stop disabling it.

Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: linux-kbuild@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/lkml/CAKwvOd=bcs5W6eEDXGn5ROR2EuMiSp_2e6sLFxak=KK6yLWcvA@mail.gmail.com