feat: add RISC-V 64-bit platform support

[gounthar]

Summary

• Add riscv64gc-unknown-linux-gnu to the CI release build matrix, using the same ubuntu:20.04 container + taiki-e/setup-cross-toolchain-action@v1 cross-compilation approach as the existing aarch64 and i686 targets
• Add JReleaser platform replacement (linux-riscv_64) and distribution artifact mapping
• Update README with the new supported architecture

Details

No Rust code changes needed — std::env::consts::ARCH already returns "riscv64" natively. The gap was only in the build/release pipeline.

The gcc-riscv64-linux-gnu cross-compiler is available in Ubuntu 20.04 (Focal), so the existing container image works without changes.

Closes #367

Test plan

• Verify YAML indentation and naming consistency (linux-riscv_64 follows linux-x86_64, linux-x86_32, linux-aarch_64 convention)
• Confirm riscv64gc-unknown-linux-gnu is a valid Rust target triple
• CI release workflow builds successfully for the new target

[marc0der]

Hey @gounthar, I'm happy to accept your pull requests (many coming through, I see), but we need to think this through first. It seems you haven't read our contributing guidelines, which ask for conversation on Discord before implementation. I'll hold off with merging anything until we've had that crucial chat.

[gounthar]

Of course, sorry. I saw a discussion started by another contributor on Discord, but there were no replies, so I mistakenly assumed that nobody was against it. 🤷

[marc0der]

Yeah sorry, I've been super busy, but I've answered now. Happy to take the chat there if you like. I see you've been burning them tokens 🔥 😆

[marc0der]

Going to merge this first one, because it can't do any harm. We'll be careful to merge and deploy the related PRs to ensure that we don't break anything.

[gounthar]

Thank you so much!