[Bug]: Cannot define custom `coreutils_toolchain`

### What happened?

`coreutils_toolchain` isn't publicly exported, however users may want to define a custom `coreutils_toolchain` because of some esoteric CPU architecture

For example, being able to define custom toolchains would help solve some of these problems:
* https://github.com/aspect-build/bazel-lib/issues/701
* https://github.com/aspect-build/bazel-lib/issues/671
* https://github.com/aspect-build/bazel-lib/issues/519

I'm currently using a patched version which exports the toolchain so we can use it in a definition. We should also consider doing this for other binaries as well (tar, etc.). If PRs are welcome, I can open a PR to implement this

### Version

Development (host) and target OS/architectures: ARM / x86 centos 7

Output of `bazel --version`:
```
Bazelisk version: v1.17.0
Build label: 7.0.0
Build target: @@//src/main/java/com/google/devtools/build/lib/bazel:BazelServer
Build time: Mon Dec 11 16:50:16 2023 (1702313416)
Build timestamp: 1702313416
Build timestamp as int: 1702313416
```

Version of the Aspect rules, or other relevant rules from your
`WORKSPACE` or `MODULE.bazel` file:
```
faaada2eaa08de480f0db4fd1fcddead6aaed177 for bazel-lib
```

Language(s) and/or frameworks involved:
aspect_bazel_lib, aspect_rules_js

### How to reproduce

_No response_

### Any other information?

_No response_

Start by locating the definition and exports for `coreutils_toolchain` in bazel-lib, then inspect how related binary toolchains such as `tar` are exposed. Confirm that a user can define a custom toolchain for an unusual CPU architecture, and run the repository's relevant Bazel tests to verify the public API.

Written by the indexing model from the issue text.