Add support for arm64_mac architecture

[hashicorp-cla-app[bot]]

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[jaysoffian]

The approach looks good to me, I left some questions/remarks on the code for now, mostly because the Architecture was not updated for all the builders/post-processors (not sure if they all support it though), but aside from that, the code looks good to me.

As far as the builders go, this PR updates chroot and ebssurrogate which are the only builders where you'd potentially mount an AMI from a different architecture than the host, so they are the only ones that have an architecture configuration item. As far as the other builders:

• ebs: customizes the running AMI,so the architecture stays the same as the host.
• instance: not compatible with macOS EC2 instances which can only boot from an EBS volume.
• ebsvolume: doesn't build an AMI.

With respect to post-processors, this plugin has only the import plugin:

The Packer Amazon Import post-processor takes an OVA artifact from various builders and imports it to an AMI available to Amazon Web Services EC2.

Amazon requires EC2 Mac instances to boot from one of their AMIs (or a customized version thereof). I don't think you can start with an arbitrary OVA artifact. So I don't think supporting the two Mac architectures in the import post-processor makes sense.