Improve tips when installing tier 3 targets such as wasm32-wasip2

[AaronFriel]

Verification

• I searched for recent similar issues at https://github.com/rust-lang/rustup/issues?q=is%3Aissue+is%3Aopen%2Cclosed and found no duplicates.
• I am on the latest version of Rustup according to https://github.com/rust-lang/rustup/tags and am still able to reproduce my issue.

Problem

Installing the wasm32-wasip2 target on stable fails consistently with a confusing error message.

Steps

The command and error is:

$ rustup target add wasm32-wasip2 --toolchain stable
error: toolchain 'stable-x86_64-unknown-linux-gnu' does not contain component 'rust-std' for target 'wasm32-wasip2'; did you mean 'wasm32-wasip1'?
note: not all platforms have the standard library pre-compiled: https://doc.rust-lang.org/nightly/rustc/platform-support.html

There are a few inconsistencies:

1. That link always goes to nightly, not stable documentation.
2. Both nightly platform support and stable platform support pages show the same table for wasm32-wasip2 support: rust-std is supported.
3. The Rustup component availability page "Rustup packages availability on ..." shows rust-std as an available component for wasm32-wasip2.

Possible Solution(s)

No response

Notes

No response

Rustup version

$ rustup --version
rustup 1.26.0 (1980-01-01)
info: This is the version for the rustup toolchain manager, not the rustc compiler.
info: The currently active `rustc` version is `rustc 1.80.1 (3f5fd8dd4 2024-08-06)`

Installed toolchains

$ rustup show
Default host: x86_64-unknown-linux-gnu
rustup home:  /home/friel/.rustup

installed toolchains
--------------------

stable-x86_64-unknown-linux-gnu (default)
beta-x86_64-unknown-linux-gnu
nightly-x86_64-unknown-linux-gnu

installed targets for active toolchain
--------------------------------------

wasm32-wasi
x86_64-unknown-linux-gnu

active toolchain
----------------

stable-x86_64-unknown-linux-gnu (default)
rustc 1.80.1 (3f5fd8dd4 2024-08-06)

OS version

$ cat /etc/lsb-release 
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=24.04
DISTRIB_CODENAME=noble
DISTRIB_DESCRIPTION="Ubuntu 24.04 LTS"

[rami3l]

I am on the latest version of Rustup according to https://github.com/rust-lang/rustup/tags and am still able to reproduce my issue.

@AaronFriel Thanks for filing this issue! However, you'll need to make sure this is reproducible on Rustup v1.27.1 before ticking the box above.

So far, wasip2 is tier 3, so even if std is supported for this target, no precompiled artifact is currently available for it on the server, so you'll need to compile it yourself. Please refer to https://stackoverflow.com/a/67787259 to see how to do this on your machine.

Thus, I consider the link in the message above to be mostly correct.

On the other hand, given the current usage of platforms in the current workspace, we can add a special case of this message for tier 3 targets.