CI: Add aarch64-apple-darwin jobs to GitHub Actions
CI: Add aarch64-apple-darwin jobs to GitHub Actions

Adds cargo build and cargo-c build jobs to the macOS build matrix for the aarch64-apple-darwin target (for Apple Silicon / M1).

On the release of Rust 1.49 all toolchains can be switched back to stable (and it can be removed from the build matrix).
xiph/rav1e,xiph/rav1e