Add cva6 specific riscv_instr_gen_tb_top
Created by: CoralieAllioux
This PR aims to resolve issue #1816 (closed)
The chosen solution is the same one than the other cores: adapting the riscv_instr_gen_tb_top to cva6. That module could not be extended as suggested, since it is not a class.
This has been check with VCS 2021.09: the behavior is the same than previous on our side.