cxxrtl: factor out -noproc/-noflatten from -O.

cxxrtl: factor out -noproc/-noflatten from -O.

Although these options can be thought of as optimizations, they are
essentially orthogonal to the core of -O, which is managing signal
buffering and scope. Going from -O4 to -O2 means going from limited
to complete design visibility, yet in both cases proc and flatten
are desirable.