Migrate Linux ARM64 builds from Circle CI to GitHub Actions

Remove Circle CI configuration entirely and migrate all Linux ARM64 builds
to GitHub Actions. Add 15 new Linux ARM64 build configurations to the
GitHub Actions build matrix using ubuntu-24.04-arm runners, covering:
- GCC 14 and clang 20 compilers
- Debug and Release builds
- Various sanitizers (ASan, TSan, UBSan)

Also add "(Linux Intel)" and "(Linux ARM64)" labels to all Linux builds
for clearer architecture distinction in the build matrix.

- Delete .circleci/config.yml and remove Circle CI integration
- Add Linux ARM64 configurations to .github/workflows/build.yml
- Remove Circle CI badge from README.md
- Remove Circle CI validation from check.sh

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: laurynas-biveinis