Riscv/pvm: multiplication instructions

Owner: @ole.kruger (while @thomas.pecseli is on vacation)

What

Implement MUL, MULH, MULHSU, MULHU, and MULW instructions.

Why

We need to support the RV32M and RV64M Risc-V extensions.

How

All instructions are implemented following the approach set out when implementing the REMx instructions in the same extensions.

Manually testing the MR

Run make test in the src/risc_v subdirectory. The tests for all the multiplication instructions have been enabled.

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