sentencepiece riscv64 support

[gounthar]

Tracking riscv64 wheel support for sentencepiece (no riscv64 wheels on PyPI for 0.2.2).

The native build trips abseil's subword-atomics static_assert under GCC 13, but RISE's cibuildwheel in manylinux_2_39_riscv64 (which ships GCC 14) builds it cleanly.

Build workflow: #165 (validated on native riscv64 hardware, 60 tests passing).

[gounthar]

Licensing check (per the development guide's Licensing section), for the record before merge:

I pulled the official sentencepiece-0.2.2-cp310-cp310-manylinux_2_27_x86_64...whl from PyPI and inspected its .dist-info. It carries License-Expression: Apache-2.0 in the metadata, but ships no LICENSE file, and no notices for the abseil (Apache-2.0) and protobuf (BSD-3) sources that are statically linked into _sentencepiece.so.

Our riscv64 wheels build from the same setup.py/pyproject.toml, so they will match upstream's packaging exactly, including this gap. It is an upstream omission rather than anything the riscv64 build introduces.

Flagging it rather than silently patching: the guide treats this case by case. Options are to accept parity with the upstream wheel, add a build patch that bundles the license texts into our wheels, or raise it upstream so the fix lands for every platform. Happy to do whichever the maintainers prefer.