cxxrtl: add an unsupported knob for manipulating clock trees.

cxxrtl: add an unsupported knob for manipulating clock trees.

This is quite possibly the worst way to implement this, but it does
work for a subset of well-behaved designs, and can be used to measure
how much performance is lost simulating the inactive edge of a clock.

It should be replaced with a clock tree analyzer generating safe
code once it is clear how should such a thing look like.