Arm64 Linux support

[nodakai]

Please could you support Arm64 Linux? (AWS Graviton etc.)

I could set PROTOBUF_PROTOC to apt-installed protoc but it's some extra effort to make sure to set it across multiple environments, multiple contexts (SSH, cron, sudo, machinectl, systemd service, docker, ...)

Not sure if Arm64 Mac is worth an explicit support when almost everyone can use Rosetta binary translator. But it's perhaps more popular than i386 Mac...