Fix GCC 15 Linux build in CI

[texasich]

Summary

This PR adds GCC 15 Linux coverage in GitHub Actions and fixes the current GCC 15 build failures seen in issue #778.

What changed

• added a GCC 15 Linux CI job
• updated the SentencePiece pin to a newer revision
• disabled Highway AVX10.2 for GCC 15 builds, which matches Gemma's current target usage
• adjusted artifact naming so compiler-matrix jobs do not conflict

Why

Issue #778 reports GCC 15 build failures on Linux. With this change, the failure is reproducible in CI and the GCC 15 job now completes successfully.

Notes

• Gemma already does not rely on AVX10.2-specific paths in its own x86 target configuration
• the Highway change is limited to GCC 15 builds
• artifact naming was updated to support multiple Linux compiler entries in the same workflow run

Refs #778

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.