sentencepiece: build wheels for riscv64

[gounthar]

Adds build-sentencepiece.yml so sentencepiece riscv64 wheels build on the native ubuntu-24.04-riscv runner via cibuildwheel, and publish to the GitLab index on main.

This is the build workflow that the auto-generated #157 asks for ("No build workflow found at .github/workflows/build-sentencepiece.yml"). sentencepiece is already listed in ci_scripts/packages.txt, so no manifest change is needed here.

Shape

Mirrors build-numpy.yml:

• checks out google/sentencepiece at the requested tag (submodules: true, the C++ core is vendored)
• points cibuildwheel at the python/ subdirectory via package-dir, since that is where setup.py/pyproject.toml live
• matrix: cp312, cp313, cp314, cp314t
• publish job is gated on main and reuses the shared publish-to-gitlab action

Wheels are also tested inline: cibuildwheel installs each freshly built wheel into an isolated venv and runs the package's own pytest suite against it (CIBW_TEST_SOURCES + CIBW_TEST_COMMAND). Two files are skipped: gen_stubs_test.py imports a repo-level helper that is not part of the copied test tree, and clean_sentencepiece_test_manual.py is a manual cleanup script rather than a pytest module.

Verification

Built and tested on a native riscv64 board (BananaPi BPI-F3), cp312, cibuildwheel 4.1.0, manylinux_2_39_riscv64:

60 passed, 1 warning in 281.29s

produced sentencepiece-0.2.2-cp312-cp312-manylinux_2_39_riscv64.whl.

Refs #164

Trigger: sentencepiece:0.2.2