ci: add GCC and Clang build matrix

[w3lld1]

Summary

• I converted the Alpine build job into a compiler matrix covering GCC and Clang.
• I pass the selected CC and CXX explicitly to Meson for each matrix entry.
• I kept both builds on the existing musl/libstdc++/BFD environment so compiler differences are isolated.

Validation

• actionlint .github/workflows/ci.yaml
• YAML matrix assertions for both compiler entries and their build environment
• Alpine package/index check for g++, clang21, clang-21, and clang++-21
• git diff --check upstream/master...HEAD

I deferred the complete container builds to GitHub Actions because the local Docker socket is unavailable.

Fixes #534