kbuild: factor out OBJECT_FILES_NON_STANDARD check into a macro

The OBJECT_FILES_NON_STANDARD check is quite long.

Factor it out into a new macro, objtool-enabled, to not repeat it.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>