ci: add riscv64 target to linux wheel build matrix

[gounthar]

Summary

Add riscv64 to the maturin-action linux build matrix so that linux_riscv64 wheels are built and published to PyPI.

maturin-action cross-compiles via riscv64gc-unknown-linux-gnu cargo target — no QEMU or additional infrastructure needed.

Evidence

• Tested wheel: textual_speedups-0.2.1-cp313-cp313-manylinux_2_34_riscv64.whl
• Built natively on BananaPi F3 (SpacemiT K1, rv64imafdcv, 8 cores @ 1.6 GHz)
• Build time: ~2 min
• Imports and passes basic smoke test on riscv64

Context

• riscv64gc-unknown-linux-gnu is a Rust tier-2 target with std available
• maturin-action handles cross-compilation transparently
• RISC-V hardware is shipping (SiFive, SpacemiT, Sophgo SG2044)

Fixes #5

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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]

Thanks for merging!