rules: cross-arch dh_shlibdeps, point at /usr/lib/<DEB_HOST_MULTIARCH>/

crossbuild-essential-arm64 transitively installs libc6:arm64 et
al. (Depends chain: crossbuild-essential-arm64 → gcc-aarch64-
linux-gnu → libc6-dev:arm64 → libc6:arm64), placing the runtime
.so files in /usr/lib/aarch64-linux-gnu/. The non-dev libs
(libc.so.6, libm.so.6, etc.) live there, not in /usr/lib/
which is what dh_shlibdeps searches by default.

dpkg-shlibdeps accepts -l<dir> for additional library search
paths. We compute the right path via dpkg-architecture -aarm64
-q DEB_HOST_MULTIARCH (which returns 'aarch64-linux-gnu'
without choking on the CC mismatch warning, since it's a pure
query), and pass /usr/lib/$HOST_MULTIARCH/ as -l. Falls back
to a literal aarch64-linux-gnu if dpkg-architecture isn't
installed (it always is on a Debian build host).

debian/rules

@@ -126,12 +126,29 @@ override_dh_auto_test:
 	# client). Build smoke is sufficient for the package.
 
 override_dh_shlibdeps:
-	# The runner image installs crossbuild-essential-arm64, which
+	# In a cross-build (DEB_HOST_ARCH != DEB_BUILD_ARCH), the
-	# provides libc6-dev:arm64 et al. plus dpkg-cross — enough
+	# system's arch-specific lib dir is at
-	# for dh_shlibdeps to resolve the arm64 binaries' NEEDED
+	# /usr/lib/<DEB_HOST_MULTIARCH>/. crossbuild-essential-arm64
-	# entries against the system's shlibs cache directly. No
+	# has transitively installed libc6:arm64 et al. there (it's
-	# stage dir, no -l hack: standard dh_shlibdeps just works.
+	# a Depends chain: crossbuild-essential-arm64 → gcc-aarch64-
-	dh_shlibdeps
+	# linux-gnu → libc6-dev:arm64 → libc6:arm64). Pass that
+	# path via -l so dpkg-shlibdeps can resolve the arm64
+	# binaries' NEEDED entries (libc.so.6, libstdc++.so.6,
+	# libfontconfig, etc.) against the on-host shlibs cache.
+	if [ -n "$(DEB_HOST_ARCH)" ] && [ "$(DEB_HOST_ARCH)" != "$(DEB_BUILD_ARCH)" ]; then \
+	    HOST_MULTIARCH=$$(dpkg-architecture -a$(DEB_HOST_ARCH) -q DEB_HOST_MULTIARCH 2>/dev/null) || \
+	        HOST_MULTIARCH=aarch64-linux-gnu; \
+	    HOST_LIB_DIR=/usr/lib/$${HOST_MULTIARCH}; \
+	    if [ -d "$$HOST_LIB_DIR" ]; then \
+	        echo "  --> dh_shlibdeps: cross-arch $$HOST_LIB_DIR (DEB_HOST_ARCH=$(DEB_HOST_ARCH))"; \
+	        dh_shlibdeps -- -l"$$HOST_LIB_DIR"; \
+	    else \
+	        echo "  --> dh_shlibdeps: WARN cross-arch dir $$HOST_LIB_DIR missing"; \
+	        dh_shlibdeps; \
+	    fi \
+	else \
+	    dh_shlibdeps; \
+	fi
 
 override_dh_strip:
 	# dh_strip disabled entirely. See MEMORY.md / AGENTS.md for the