Hi!
Please apply the attached patch.  It adds support for the x32 architecture.
This uses in every case amd64 code paths, as x32 in an ABI atop the amd64
kernel.

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/