Clang and AArch64 backend patches to support arm v8 Neon

[Tim Northover]

Hi Ana,

> Tim (and anyone else interested), can you do the review of the rebased
> patches and merge them?

I think the main thing we learned between then and now is that tests
will need a full aarch64-linux-gnu triple to avoid breaking on
unsupported platforms (otherwise they try to use aarch64-apple-darwin
or aarch64-microsoft-win32 or something and go horribly wrong).

Other than that, I've had a look for areas that are substantially
different now and I didn't spot anything. I'm still happy for them to
go in.

Cheers.

Tim.