macOS: Add support for aarch64-*-darwin* sysname

macOS: Add support for aarch64-*-darwin* sysname

As of libtool 2.4.7, the supplied config.guess file modifies
the sysname returned on macOS arm64 from arm-*-darwin* to
aarch64-*-darwin*. As a result, trying to build OpenAFS on
macOS arm64 fails with the following message:

  configure: error: An AFS sysname is required

To avoid this problem, added support for aarch64-*-darwin*.

Change-Id: Id00d5a749ffb9ef49f0ebba6d8b9a47c5c3a9d6a
Signed-off-by: Ganesh Chaudhari <gangovind@in.ibm.com>
Reviewed-on: https://gerrit.openafs.org/15149
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>