Skip building the gdm3 binary package on s390x

GDM requires GNOME Shell, which provides its GUI, but GNOME Shell
is unavailable on s390x.

Our previous attempt to avoid building an uninstallable gdm3 package
on s390x involved a spurious build-dependency on gjs, but mozjs60 and
gjs now work well enough on s390x to pass most of their own tests.
However, GNOME Shell still fails tests on s390x, and in any case is
not appropriate for mainframe-class machines that do not normally have
a GPU or a local display.

Continuing to build libgdm binaries for s390x, as opposed to forcing the
entire source package to FTBFS on s390x, means we can continue to build
gnome-panel without special hacks, allowing s390x users to install the
GNOME 2-based GNOME Flashback environment if they particularly want a
GNOME-based desktop on their mainframes.

Closes: #935699