fix(toolchains): add musl list of freethreaded runtimes (workspace)

[rickeylev]

The musl entry got lost as part of a refactoring. Add it back to the platforms
that generate freethreaded variants.

Fixes #3286

[gemini-code-assist]

Summary of Changes

Hello @rickeylev, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a regression where the 'musl' platform was no longer correctly identified for generating freethreaded Python runtimes with specific optimization flags. By reintroducing the 'musl' entry, it restores proper build configurations for 'musl'-based Linux systems, ensuring compatibility and optimized performance.

Highlights

• Musl Support Restoration: Reinstated the 'musl' platform entry in the freethreaded runtime generation logic, which was inadvertently removed during a previous refactoring.
• PGO+LTO for Musl: Ensured that the 'x86_64-unknown-linux-musl' platform correctly receives 'pgo+lto' optimization flags for freethreaded variants.

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[gemini-code-assist]

Code Review

This pull request addresses an issue where the musl entry was missing for freethreaded runtimes. The change correctly adds x86_64-unknown-linux-musl to the platform dictionary. The placement of the new entry maintains the existing alphabetical order of the keys. The fix is straightforward and appears correct. I have no further suggestions.

[rickeylev]

While creating tests for this, I noticed some weird things with toolchain registration. I think there's a bug in the order that python_register_toolchains() registers things for workspace and in the toolchain constraints, the net effect being you can never match the musl or freethreaded toolchains (I think Yulia filed a bug for this, too).

I think this fix also only affects workspace -- bzlmod is using what's in MODULE.bazel, which I think is also omitting musl. I don't see a musl-freethreaded toolchain showing up there

[rickeylev]

Ahh, I think I see -- we don't have a musl-freethreaded build built into our versions.bzl file. There are such builds available. JAX must be patching versions.bzl and adding such an entry themselves (or equivalent).

Ugh, versions.bzl is such a mess.