The current `run` step of the symbiyosys backend generates a `sby` command which includes the hardcoded `-d build` argument. However, this command line argument is incompatible with multiple tasks.

This fix removes the hardcoded `-d` argument and introduces a new `extra_options` tool option for the symbiyosys backend, which can be used to pass additional arguments for the `sby` command (e.g. `-d build`, if necessary).

The example usage of the `extra_options`:
```
   symbiyosys:
     tasknames:
       # A list of task names to pass to sby. Defaults to empty (in which case
       # sby will run each task in the .sby file)
       - my_proof
     extra_options:
       # A list of extra command line arguments to sby. Defaults to empty.
       - -d
       - build
```

…nto working-set

#362