Build for new RISC-V PVM target

[vapourismo]

As of this commit, the Smart Rollup Kernel SDK has stubs for the new RISC-V supervising target (aka linux-musl). I know you need this target for Jstz to move to a new JS runtime.

While you can't run kernels produced using this target yet, it makes sense to incorporate them into CI already and ensure they are built.

Here is an example of how to produce such a kernel.