RISC-V: Constrain capabilities on Posix stepper methods

Part of RV-157

Part of RV-152

Part of RV-46

What

Fix constraints on Posix stepper methods.

Why

More precise constraints across the code base allow us to limit the capabilities of PVM state methods thereby making them more correct (e.g. no reads or writes when not allowed).

Manually Testing

make -C src/riscv all

Benchmarking

No performance impact has been observed on the reference machine.

Tasks for the Author

• All Linear issues that relate to this MR were linked using magic words (e.g. part of, relates to, closes)
• There is no dead code and other spurious artefacts in my changes
• All newly introduced public functions, methods and types are documented
• Documentation of changed functions, methods and types is up-to-date
• There are tests which cover bugs which have been fixed, if any
• Reasonable effort has been made towards making CI pass
• Performance has been benchmarked and populated in the table above
• All the above was completed before assigning this MR to reviewers