macos-arm64 build

[jmatsushita]

Hi there,

It would be great if zephyr had a macos-arm64 (aka aarch64-darwin) build like purescript itself does it would allow to provide this architecture for the easy-purescript-nix tooling which doesn't currently support it because of https://github.com/justinwoo/easy-purescript-nix/blob/master/zephyr.nix#L20

Cheers,

Jun