RFC: Promote AArch64 to be built by default

[Tim Northover]

Hi all,

We'd like to promote the AArch64 backend to being built by default. I
know it's very soon after the initial upload, but we think the backend
is in a good position and should be stable.

We'd very much like to get it tested in more diverse environments
provided by the buildbots, and are committed to cleaning up any
fallout as soon as possible, or as a last resort reverting the
"enable" commit.

As before, this doesn't affect our attitude to more reviews: they
would be very welcome.

Does anyone object to me making the necessary changes?

Cheers.

Tim.