Currently crosscompiling for linux aarch64 (ARM64) is not supported.

This seems strange, since e.g. "arm64-darwin" is supported and IMHO that's a more complicated setup than arm64 linux, where docker can be utilized to provide the necessary crosscompilation toolchain.

We'd need this for gRPC and protobuf to be able to provide arm64 linux native gems.
grpc/grpc#26391