feat(toolchain): support freethreaded toolchains

[aignas]

Before this PR freethreaded toolchains were not possible to be used,
this adds the minimum plumbing to get the things working. Coverage
support is also added.

Whilst at it:

• Add plumbing to print checksums only for a particular python version.
• Bump the remaining toolchain versions that used to use the 20241008 release
• Pass around the loaded platform list so that we are only defining toolchains for the platforms that we have loaded the hermetic toolchain for.

Tested:

$ bazel run --//python/config_settings:python_version=3.13.0 --//python/config_settings:py_freethreaded="yes" //python/private:current_interpreter_executable
...
Python 3.13.0 experimental free-threading build (main, Oct 16 2024, 03:26:14) [Clang 18.1.8 ] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

Closes #2129.
Work towards #2386.

[vfdev-5]

Seems like this PR is working to build XLA with Python free-threading using the following command:

bazel build \
    --@rules_python//python/config_settings:py_freethreaded="yes" \
    --define=use_enabled_free_threading=true \
    --test_output=all --spawn_strategy=sandboxed \
    --override_repository=rules_python=/rules_python \
    //xla/...

It downloads however two versions of python:

ls /root/.cache/bazel/_bazel_root/83d0ab0b1a11dcb5adba61b76e88291b/external/python                                                                                                         
...
python_x86_64-unknown-linux-gnu-freethreaded/                                                                            
python_x86_64-unknown-linux-gnu/
...

I assume that free-threaded version is used for building python extensions...

[aignas]

@vfdev-5, I am not fully sure about why you have the toolchain in your cache. Could you please check it with bazel query or use --toolchain_resolution_debug=python to check?

[vfdev-5]

In my tests I deleted completely the cache and run xla build from the very beginning. In the console I saw it downloading python for platform x86_64-unknown-linux-gnu and after that x86_64-unknown-linux-gnu-freethreaded.
I'll retry with your suggestions to figure out

[aignas]

Done with the self-review.

[groodt]

LGTM

[rickeylev]

So if I remember the toolchain generation logic, then all one has to do is set the flag, and it'll trigger downloading and usage of the free-threaded interpreter?

Otherwise just some small comments, LGTM

[rickeylev]

Just checking/curious: I'm guessing the libX.Y* pattern works because the extracted standalone interpreters only contain appropriately relevant .so files?

[aignas]

This is for exclusion, that is why libX.Y*.so is just as good as libX.Y.so.