Hi!
Please apply the attached patch.  It defines kernel selectors for the x32
architecture.  As x32 is a new ABI atop the amd64 kernel, this patch
symlinks kernel/amd64.sh to kernel/x32.sh, to keep them in sync in the
future.  The patch can be applied directly to the d-i/base-installer 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/

Oops, the patch wrongly links to kernel/amd64.sh rather than just amd64.sh
I haven't noticed this in testing as the version uploaded to my private repo
was patched manually.  A fixed version is attached.