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.
Closesrust-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`
6647be9364