Estimate the power consumption of a C program without hardware
What you are trying to do won't really work.  First a "C program" doesn't consume a particular amount of power.

A particular program performing a particular task on a particular processor may cause a reasonably measurable increase in power draw by that processor, or not.  The power draw of a processor will only change significantly if it would otherwise go into some kind of low power mode if the "program" wasn't running.  On many small systems, not doing one thing only means doing more of other things.  The only effect of running a particular piece of code might be to respond with higher latency to new events, for example.

Even if you can measure a reasonably repeatable power increase in one processor due to running a particular program, that is little indication of anything useful for the same task performed some other way using different technology.  You really should not expect the power increase due to a RPi running a particular program to have a meaningful correlation to the power required to run a FPGA that performs the same function.