Added retire event. Moved insn_disas, insn_pc, and insn_val assignmen…

Created by: GTumbush

This pull request is to address https://github.com/pulp-platform/riscv/issues/218

1. Added a retire event.
2. Moved assignment to insn_disas, insn_pc, and insn_val assignment to after retire event trigger so these variables change only when an instruction is retired. This enables a testbench to wait on the retire event and collect the results of the retired instructions. If you want to keep the assignment of these 3 variables as-is I can create separate signals.