First stab at RVFI interface

Created by: jrrk

@zarubaf This is a first stab at implementing RVFI for Ariane. Some placeholders have not been filled in yet. Review welcome. Moreover RVFI port hierarchical references will need flattening for compatibility with older synthesis tools. Also included is the direct instruction injection facility (bypassing the icache) needed by Cambridge University's TestRIG tool which provides formal (random rather than symbolic) comparison against spike or sail models. Feedback on the veracity of this implementation is also eagerly anticipated. This methodology is based on the ibex RVFI verilator support.