This adds a Verilator simulation of Ibex for use in RISC-V Compliance
Testing. In addition to ibex itself, the simulation contains a RAM and
a memory-mapped helper module for the test software to interact with the
outside world. The test framework uses this to dump a "test signature",
which is written to a certain part of the memory, and to end the
simulation. (In the future, this could be extended to include printf()
like functionality.)
