CVXIF issue_valid and commit_valid are always sent at the same clk cycle

[ZElkacimi]

CVA6 always send the issue transaction and the commit transaction at the same clk cycle. It's allowed by the cvxif Spec, but if it is hard coded it may cause problems in some cases where cva6 needs to set the commit_kill (currently CVA6 never raise the commit_kill signal).