Add support for linux/arm64

[radeksimko]

This mimics build targets of latest Terraform (core) version, e.g.
https://releases.hashicorp.com/terraform/0.14.3/

Closes #342

[radeksimko]

I'm unsure whether the above 3 combinations are ignored intentionally, or by accident as they aren't explicitly mentioned here https://github.com/hashicorp/terraform/blob/c0e37a7ea1928918a9efc02290ecd241d396d3a6/scripts/build.sh#L20 but gox doesn't support them:
https://github.com/mitchellh/gox/blob/master/platform.go

Either way there is no point in providing LS builds if there is no corresponding Terraform build, so I opted to ignore these. 🤷🏻‍♂️

[aeschright]

LGTM!

[ghost]

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