The libtool RPM installs an executable called libtool, which includes some system specific information gathered by the configure script. This information could concievably be different for different platforms, but the same noarch RPM is used for all platforms. It is true that the one package will `work' on all platforms, but it may not be correct on all platforms. I don't know if this bug has been fixed in 5.9
Red Hat systems are similar enough that libtool is safely a noarch package. Red Hat also explicitly does not include the per-arch libraries. This has been made explicit in libtool-1.3-2.