Promote aarch64-unknown-linux-gnu to Tier 1
This PR promotes the `aarch64-unknown-linux-gnu` target to Tier 1, as proposed by [RFC 2959]:
* The `aarch64-gnu` CI job is moved from `auto-fallible` to `auto`.
* The platform support documentation is updated, uplifting the target to Tiert 1 with a note about missing stack probes support.
* Building the documentation is enabled for the target, as we produce the `rust-docs` component for all Tier 1 platforms.
[RFC 2959]: https://github.com/rust-lang/rfcs/pull/2959
50086afb5d