kbuild: move -fzero-init-padding-bits=all to the top-level Makefile

The -fzero-init-padding-bits=all option is not a warning flag, so
defining it in scripts/Makefile.extrawarn is inconsistent.

Move it to the top-level Makefile for consistency.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>