Promote riscv64-unknown-linux-musl to tier 2 with host tools

[eshattow]

MCP: rust-lang/compiler-team#982

Implements #156191

[rustbot]

This PR changes how LLVM is built. Consider updating src/bootstrap/download-ci-llvm-stamp.

Warning

If you are changing how CI LLVM is built or linked, make sure to bump
src/bootstrap/download-ci-llvm-stamp.

cc @jieyouxu

These commits modify compiler targets.
(See the Target Tier Policy.)

Some changes occurred in src/doc/rustc/src/platform-support

cc @Noratrieb

[rustbot]

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @marcoieni (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

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Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: infra-ci
• infra-ci expanded to Kobzol, Mark-Simulacrum, jdno, jieyouxu, marcoieni
• Random selection from Mark-Simulacrum, jdno, jieyouxu, marcoieni

[rustbot]

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