Makefile: Link with -z noexectack

When moving to gcc-12.2 we started trying to quiet some of the new
linker warnings, that are not relevant to us. However, a
misunderstanding of the mechanics at play meant that I intentionally
omitted passing -z noexecstack to the linker, when we do need to. Add
this flag and in turn remove warnings from the linker.

Fixes: 1e1c51f8 ("Makefile: link with --no-warn-rwx-segments")
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>