RISC-V: add skeleton for PVM Execution Environment

What

Provides a skeleton for the PVM SBI execution environment.

Why

This is the first step toward implementing the PVM execution environment, to provide PVM functionality in RISC-V.

How

This skeleton implements the ExecutionEnvironment trait, and provides a structure for SBI handling to be implemented. Environmental calls (calls from machine code) are currently fatal, SBI functionality is unimplemented.

Dead code as part of the skeleton is currently allowed and will be removed as the PVM execution environment is implemented.

Manually testing the MR

Currently, there exists no functionality or meaningful state. Testing should be added as functionality is implemented, and for now the code should not cause any new warnings or errors in the normal testing pipeline.