|In Debian /lib64/ should just be a symlink to /lib/, due to a packaging bug in libc6 that ceased to be the case some time ago…
So the correct approach to return to the expected behaviour would be to start from the aptosid live CD, mount your / under /mnt/ and:
if your system wasn't fully updated at the time you broke it, you may have to adapt the upstream version number of ld-*.so. Fortunately this works, just because Debian policy forbids /lib64/ to be used.
# mkdir --mode=0755 /mnt/lib64
# ln -fs /lib/x86_64-linux-gnu/ld-2.13.so /mnt/lib64/ld-linux-x86-64.so.2