scanner: avoid executable stack

Before this patch:
$ scanelf -lpqe ./wayland-scanner
RWX --- ---  ./wayland-scanner

That indicates the stack is executable, which is a bad thing for
security. Wayland-scanner does not actually need an executable stack, it
is just an oversight from using an .S file in the sources.

Add a special incantation in dtddata.S to make it not cause the stack to
become executable.

Reported-by: Mart Raudsepp <leio@gentoo.org>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Tested-by: Mart Raudsepp <leio@gentoo.org>