(partially) fix RVFI integration issues

This PR resolves SV compile errors and warnings flagged by dsim:

• bhv/cv32e40p_rvfi.sv: nonblocking assignments to automatic variables are not allowed (object variables are automatic by default). Dsim won't compile this but I expect VCS does.
• bhv/cv32e40p_tb_wrapper.sv: dsim warns about several width mismatches. I did not resolve this, but they are flagged with FIXMEs.