add helper for discovering and aliasing system architecture

[ca-scribner]

Several of our charms do something like this for architecture discovery:

        arch = platform.processor()
        if arch in ["x86_64", "amd64"]:
            self._arch = "amd64"
        elif arch in ["aarch64", "arm64", "armv8b", "armv8l"]:
            self._arch = "arm64"
        else:
            self._arch = arch

Examples (note that they don't all map to the same final arch alias names):

• cos-proxy
• loki_push_api lib
• grafana agent operator pr

We should decide on one common method and include it here as a helper.