Decouple DynInst from build

[bmihaila]

As there is already a way to feed a CFG from JSON to the fingerprinter it would be great to make the DynInst code parts there optional. This would make building the whole infrastructure much easier for cases where people use a different disassmbler/CFG generator as it would skip the whole DynInst build - which is complex and impossible on some architectures.

Seems that this file https://github.com/googleprojectzero/functionsimsearch/blob/master/disassembly/disassembly.hpp would need to refactored to abstract away the DynInst parts and make it compile optional.