ci: add riscv64 native build workflow

[gounthar]

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What does this PR do?

Adds a non-blocking CI job that builds Mosquitto on native riscv64 hardware using RISE Project GitHub Actions runners (ubuntu-24.04-riscv). The job uses continue-on-error: true so it does not block existing CI.

Evidence

Validated on RISE native riscv64 runner:

• Build time: 1m38s
• CI run: https://github.com/gounthar/mosquitto/actions/runs/23343365285/job/67902292909
• All binaries build and run: mosquitto (2.1.2), mosquitto_pub, mosquitto_sub

Also validated on BananaPi F3 (SpacemiT K1, rv64gc, 8 cores @ 1.6 GHz):

• Build time: 1m30s wall, 9m CPU

No source changes needed. The workflow installs build dependencies and runs the standard cmake build.

Related

• Closes Add riscv64 to CI build matrix #3545
• Related to Missing riscv64 snap #3224 (riscv64 snap)

[gounthar]

A note on the ubuntu-24.04-riscv runner: this is a native riscv64 GitHub Actions runner provided by the RISE Project (https://riseproject.dev), an initiative by RISC-V International to support the open source ecosystem.

To enable it on the eclipse-mosquitto org, a maintainer would install the RISE runners GitHub App: https://github.com/apps/rise-risc-v-runners

It's free for open source projects and several projects already use it (numpy, Apache Arrow, llama.cpp). The runner is a Scaleway EM-RV1 bare-metal riscv64 server running Ubuntu 24.04.

Without the app installed, this workflow would simply show as "queued" with no available runner. The continue-on-error: true ensures it never blocks other CI jobs regardless.

If installing the app isn't desirable, an alternative would be using QEMU emulation via docker/setup-qemu-action on a standard runner, though build times would be significantly longer.