Implement the MCP 932: Promote riscv64a23-unknown-linux-gnu to Tier 2 Implement the [MCP 932](https://github.com/rust-lang/compiler-team/issues/932): Promote riscv64a23-unknown-linux-gnu to Tier 2 without host tools. Closes rust-lang/rust#148353. Changes: - Update target tier from 3 to 2 in target specification - Update platform documentation - Add CI/CD support for automatic building and distribution via rustup r? jieyouxu cc `@davidtwco` `@Noratrieb`