Hi!
Please apply the attached patch.  It adds a default disk label type for the
x32 architecture.  Without this patch, the user gets an error message and
has to choose GPT/MSDOS on his own.  The patch can be applied directly to
the d-i/partman-partitioning git repository.

If you want to test, as the patch-set for x32 in d-i involves around twenty
packages, you'll want ready packages from the repository at debian-x32.org.
Complete d-i isos are available at http://debian-x32.org/#debian-installer
while debs/udebs/modified sources at http://ftp.debian-x32.org/debian/