BinaryPlatforms: Recognize MacOS(:aarch64)

[Keno]

Also normalize arm64 -> aarch64, c.f.
https://lists.gnu.org/archive/html/config-patches/2020-07/msg00002.html

[giordano]

MacOS() still defaults to x86_64, kept for backward compatibility I guess?

[staticfloat]

Yes, it's going to be a while before that's changed, I think. :P

[giordano]

Sure, I was not thinking about changing the default, but rather about the fact other platforms with multiple possible architectures, like Linux and Windows, don't have a default constructor without the specification of the architecture. I assume removing the MacOS() method would be breaking though.

[staticfloat]

Indeed; hindsight is 20:20.

That being said, if we think it would be better, I have no qualms about breaking build_tarballs.jl files. It's a very easy fix, and Yggdrasil contains a high percentage of what we care about.