Unrolled build for rust-lang#135992

Rollup merge of rust-lang#135992 - madsmtm:new-target-docs, r=jieyouxu

Improve documentation when adding a new target

rust-lang#133631 (comment) shows that it can be a bit difficult process-wise to add a new target.

I've added a bit of text to the docs, suggesting that users add the target defintion/spec first, and later work on `std` support.

I also found that we have two places where we document how to add a new target. I've linked these for now, but they should probably be merged somehow in the future.

`@rustbot` label A-docs
r? compiler
CC `@workingjubilee` who's worked a lot on target specs IIRC.

Author: rust-timer

src/doc/rustc-dev-guide/src/building/new-target.md

@@ -4,8 +4,13 @@ These are a set of steps to add support for a new target. There are
 numerous end states and paths to get there, so not all sections may be
 relevant to your desired goal.
 
+See also the associated documentation in the
+[target tier policy][target_tier_policy_add].
+
 <!-- toc -->
 
+[target_tier_policy_add]: https://doc.rust-lang.org/rustc/target-tier-policy.html#adding-a-new-target
+
 ## Specifying a new LLVM
 
 For very new targets, you may need to use a different fork of LLVM

src/doc/rustc/src/target-tier-policy.md

@@ -122,12 +122,23 @@ To propose addition of a new target, open a pull request on [`rust-lang/rust`]:
   r? compiler
   ```
 
+See also the documentation in the `rustc-dev-guide` on [adding a new target to
+`rustc`][rustc_dev_guide_add_target].
+
+Note that adding a new target that wants to support `std` would transitively
+require `cc` and `libc` support. However, these would like to know about the
+target from `rustc` as well. To break this cycle, you are strongly encouraged
+to add a _minimal_ `#![no_core]` target spec first to teach `rustc` about the
+target's existence, and add `std` support as a follow-up once you've added
+support for the target in `cc` and `libc`.
+
 [tier3example]: https://github.com/rust-lang/rust/pull/94872
 [platform_template]: https://github.com/rust-lang/rust/blob/master/src/doc/rustc/src/platform-support/TEMPLATE.md
 [summary]: https://github.com/rust-lang/rust/blob/master/src/doc/rustc/src/SUMMARY.md
 [platformsupport]: https://github.com/rust-lang/rust/blob/master/src/doc/rustc/src/platform-support.md
 [rust_compiler_team]: https://www.rust-lang.org/governance/teams/compiler
 [`rust-lang/rust`]: https://github.com/rust-lang/rust
+[rustc_dev_guide_add_target]: https://rustc-dev-guide.rust-lang.org/building/new-target.html
 
 ## Tier 3 target policy