RISC-V: PVM module for the rollup node

What

Provide skeleton PVM implementation in lib_risc_v and use this in lib_sc_rollup_node. This includes only compute_step_many, the execution status and using the state provided by lib_risc_v. It doesn't include proofs and a lot of other things.

Why

We have to start somewhere. There are files related to Risc-V in the protocol. Those files contain stub implementations that we will eventually have to replace. This MR starts this.

How

There is a simple PVM module in lib_risc_v that we can build on to incrementally remove stub implementations from the protocol and replace with the implementations we provide in our library.

Manually testing the MR

Nothing to test yet.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR