Build top module as 'dut' in Verilator and set it as top-level.

Build top module as 'dut' in Verilator and set it as top-level.

When building a design with PicoRV32 we end up with multiple top-level
modules and Verilator becomes confused as to which is the right one.
This change ensures the dut.v generated by the sim build process has
it's top-level name set to 'dut' and that verilator is invoked with this
name.