Comment 2 for bug 1263756

libdebian-installer_0.89 in sid has arm64 support that we'll need to sync.

Note that this change will also fix the highbank/midway subarch detection, so I've prepared a second MP for base-installer that fixes up our kernel detection there as well (lp:~dannf/ubuntu/trusty/base-installer/arm64+calxeda-subarches).