Update cross-compilation to aarch64 target

[samderanova]

Resolves #24.

Changes

• Update .cargo/config.toml to use the 64-bit target aarch64-unknown-linux-gnu and its respective linker (aarch64-unknown-linux-gnu-gcc for MacOS and aarch64-none-linux-gnu-gcc for other OS's).
  • The MacOS based one is commented out so that Mac users can simply uncomment and use this linker instead when compiling.
• Update README.md to instruct the reader to download the new target.
• Update GitHub workflow to use new target.

Testing

• Attempt to build a release version of the program, either via cross build --release or cargo build --release. Both should run without errors.
• Copy the executable located at target/aarch64-unknown-linux-gnu-gcc/release/pod-operation to the Raspberry Pi. Then log into the Pi, either via SSH or other means.
• Make the file executable on the Pi by running chmod +x ./pod-operation.
• Run the executable by running ./pod-operation. Both the blink demo and the socket server should start up properly.

[taesungh]

Thanks for fixing this up, will test on the Pi on the pod in lab with the new 64-bit OS but trusting it works otherwise.