Add an arm64 autobuilder (linux).

This is needed to run in termux on some android devices. And of course there are arm64 servers, although the armel build probably also works on them.

Status: Builds fine on arm64, but needs an autobuilder. Building under emulation could be done, or a scaleway arm64 server, which would be a $5/month expense. Or, perhaps someone has an arm64 that could host the autobuilder? --Joey

Currently running release builds for arm64 on my phone, but it's not practical to run an autobuilder there. --Joey