build-rootfs: Run ldconfig via QEMU

Using the host's `ldconfig` doesn't work, as it only considers libraries
for the same machine type as itself.  THus, we have to execute the
target's copy, which means running it via QEMU userspace emulation.

build-rootfs.sh

@@ -76,7 +76,7 @@ if [ -f /mnt/gentoo/etc/udev/hwdb.bin ]; then
     mv /mnt/gentoo/etc/udev/hwdb.bin /mnt/gentoo/usr/lib/udev/
 fi
 
-ldconfig -r /mnt/gentoo
+qemu-${target%%-*} /mnt/gentoo/usr/sbin/ldconfig -r /mnt/gentoo
 
 rm -f /mnt/gentoo/lib/tmpfiles.d/provision.conf
 systemd-tmpfiles --root=/mnt/gentoo -E --exclude-prefix=/var --create