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]]

>> [[done]]; the current qemu based autobuilder is not ideal, often gets
>> stuck, but there's no point leaving this todo open. --[[Joey]]