ci: add riscv64 wheel builds

[gounthar]

Summary

Add linux_riscv64 wheels to the build workflow, mirroring the existing
aarch64 build job.

Changes

• New build_wheels_riscv64 job using cibuildwheel + QEMU emulation
• Targets CPython 3.10–3.14 (including free-threaded) on manylinux_riscv64
• Skips musllinux (no riscv64 image available)
• Includes Rust/Cargo environment for cross-compilation inside QEMU
• 120-minute timeout to account for QEMU + Rust compilation overhead
• Added to join_artifacts dependency list

Evidence

A tested riscv64 wheel is available in our community index:
https://gounthar.github.io/riscv64-python-wheels/simple/tiktoken/

Built natively on BananaPi F3 (SpacemiT K1, rv64imafdcv, 8 cores @ 1.6 GHz, 16 GB RAM).

Context

• manylinux_2_28_riscv64 is available in pypa/manylinux
• cibuildwheel 3.x supports riscv64 via QEMU
• Rust cross-compiles to riscv64gc-unknown-linux-gnu inside the manylinux container
• Several packages already ship riscv64 wheels on PyPI (aiohttp, yarl, multidict, regex)
• RISC-V hardware is shipping (SiFive, SpacemiT K1/K3, Sophgo SG2044)

Closes #502

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Note: this work is part of the RISE Project effort to improve Python ecosystem support on riscv64 platforms. Native riscv64 CI runners are available for free via RISE RISC-V runners.

[gounthar]

Native build verified on riscv64 hardware (BananaPi F3, SpacemiT K1, rv64gc, GCC 14.2.0): SUCCESS.

[gounthar]

Successful riscv64 build on native RISE runner (~5 min): https://github.com/gounthar/tiktoken/actions/runs/23745982111