build-rootfs: Relocate misplaced PAM libs

Some packages, notably _systemd_, place their PAM modules in
`/usr/lib64/security` instead of `/usr/lib/security`.  PAM only seems to
look in the latter location, though, so anything in the former needs to
be moved in order to be used.

build-rootfs.sh

@@ -89,6 +89,10 @@ qemu-${target%%-*} \
     -r /mnt/gentoo \
     -f /mnt/gentoo/etc/ld.so.conf
 
+if [ -d /mnt/gentoo/lib64/security ]; then
+    mv /mnt/gentoo/lib64/security/pam_*.so /mnt/gentoo/lib/security/
+fi
+
 rm -f /mnt/gentoo/lib/tmpfiles.d/provision.conf
 systemd-tmpfiles --root=/mnt/gentoo -E --exclude-prefix=/var --create