ci: add riscv64gc-unknown-linux-gnu to release artifacts

[gounthar]

This adds riscv64gc-unknown-linux-gnu to the release build matrix, so that official releases ship a pre-built Linux/RISC-V 64-bit binary.

Changes

• ci/docker/riscv64-linux/Dockerfile : new cross-compilation container (modeled on the existing aarch64-linux one, using gcc-riscv64-linux-gnu)
• .github/workflows/main.yml : new riscv64-linux matrix entry in the build job

Evidence

A fork release built with this exact configuration is available at https://github.com/gounthar/wasm-tools/releases/tag/v1.245.1 ; the wasm-tools-1.245.1-riscv64gc-unknown-linux-gnu.tar.gz artifact was verified on physical RISC-V hardware (BananaPi BPI-F3, SpacemiT K1).

Fixes #2463

[gounthar]

Note on the Ubuntu 18.04 base image: The existing Dockerfiles use Ubuntu 16.04 (Xenial) to minimize the glibc version requirement for the produced binaries. However, gcc-riscv64-linux-gnu was first packaged in Ubuntu 18.04 (Bionic), so the riscv64 Dockerfile uses 18.04 instead. 🤷

This seems acceptable to me in practice because RISC-V Linux distributions all ship glibc >= 2.27 (the 18.04 version). The earliest mainstream RISC-V distros (Fedora 33 for riscv64, Debian sid/trixie, Ubuntu 20.04+) all use glibc 2.31+, so linking against glibc 2.27 imposes no real compatibility constraint.

If maintainers prefer to keep 16.04 uniformity, an alternative would be to manually add a riscv64 cross-compiler PPA or build the cross-toolchain from source, but that adds complexity for no practical benefit given the target platform landscape.

[pchickey]

Looks OK to me but re-assigning to Alex because he has the most experience with CI and release subtleties

[alexcrichton]

Thanks!

[gounthar]

Thanks @alexcrichton for the quick review and merge! Really appreciate the responsiveness. Looking forward to riscv64 binaries shipping with the next release.