Auto merge of #148435 - ZhongyaoChen:promote-riscv64a23-to-tier2, r=jieyouxu

Implement the MCP 932: Promote riscv64a23-unknown-linux-gnu to Tier 2

Implement the [MCP 932](rust-lang/compiler-team#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`

Author: bors