Improve well known value check-cfg diagnostic for the standard library
This PR adjust the current logic for hidding the rustc/Cargo suggestion to add a value to a well-known name to exclude the standard library and rustc crates.
This is done in order to improve the contributor experience, in particular when adding a new target, which often requires adding some cfgs like `target_os` which may not be available yet in stage0.
<details>
The diagnostic code would look like this.
```text
error: unexpected `cfg` condition value: `blable`
--> library/core/src/lib.rs:369:7
|
369 | #[cfg(target_os = "blable")]
| ^^^^^^^^^^^^^^^^^^^^
|
= note: expected values for `target_os` are: `aix`, `android`, `cuda`, `dragonfly`, `emscripten`, `espidf`, `freebsd`, `fuchsia`, `haiku`, `hermit`, `horizon`, `hurd`, `illumos`, `ios`, `l4re`, `linux`, `macos`, `netbsd`, `none`, `nto`, `openbsd`, `psp`, `redox`, `solaris`, `solid_asp3`, `teeos`, `tvos`, `uefi`, `unknown`, `visionos`, `vita`, `vxworks`, `wasi`, `watchos`, and `windows` and 2 more
= help: consider using a Cargo feature instead
= help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
[lints.rust]
unexpected_cfgs = { level = "warn", check-cfg = ['cfg(target_os, values("blable"))'] }
= help: or consider adding `println!("cargo::rustc-check-cfg=cfg(target_os, values(\"blable\"))");` to the top of the `build.rs`
= note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
= note: `-D unexpected-cfgs` implied by `-D warnings`
= help: to override `-D warnings` add `#[allow(unexpected_cfgs)]`
```
</details>
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