cxxrtl: run `hierarchy -auto-top` if no top module is present.

cxxrtl: run `hierarchy -auto-top` if no top module is present.

In most cases, a CXXRTL simulation would use a top module, either
because this module serves as an entry point to the CXXRTL C API,
or because the outputs of a top module are unbuffered, improving
performance. Taking this into account, the CXXRTL backend now runs
`hierarchy -auto-top` if there is no top module. For the few cases
where this behavior is unwanted, it now accepts a `-nohierarchy`
option.

Fixes #2373.