automation: Annotate that a 32bit libc is no longer a dependency

automation: Annotate that a 32bit libc is no longer a dependency

We can't drop the 32bit libc from the existing containers, because they are
used on older Xen branches as well.

However, we can avoid the dependency being propagated into newer conainers
derived from our dockerfiles.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>