Add riscv64 support to CI and release builds

RethinkDB 2.4.4 builds on native riscv64 hardware. Architecture support is already present in the codebase (commit 4db38a8).

I attempted an automated build on a RISE riscv64 runner but the configure step failed. I am investigating the issue and will update once I have a clean build log.

No release binaries are currently attached to the v2.4.4 tag for any architecture, so riscv64 binaries may depend on broader packaging/release infrastructure plans.

If CI validation for riscv64 is of interest, the RISE project provides free ubuntu-24.04-riscv GitHub Actions runners for open-source projects. This would allow running the test suite on native hardware without QEMU.

Happy to open a PR once the build is confirmed working.