ci: Enable arm64 crosscompile job

debian/salsa-ci.yml (currently) contains this section:

   # cross building xen currently fails due to Debian bug #982406 in markdown.
   # It can be tried again when there are better chances of it building
   # successfully.
   SALSA_CI_DISABLE_CROSSBUILD_ARM64: 1

Bug 982406 was recently fixed, so I made a branch where 1 was changed to 0 and then I let Salsa's CI run.
That resulted in a failure: https://salsa.debian.org/diederik/debian-xen/-/jobs/3658861
Last part of the log:

$ eatmydata apt-get build-dep ${HOST_ARCH:+--host-architecture ${HOST_ARCH} -Pcross,nocheck} --no-install-recommends -y $aptopts .
Note, using directory '.' to get the build dependencies
Reading package lists...
Building dependency tree...
Reading state information...
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
 ocaml:arm64 : Depends: gcc:arm64 but it is not installable
               Depends: binutils:arm64 but it is not installable
E: Unable to correct problems, you have held broken packages.

I don't know yet how to fix that, but I think it would be good if we could enable as much of Salsa's CI workflow as possible.