fdpgen: replace clist implementation with generic list

fdpgen: replace clist implementation with generic list

This replaces some common/memory.h allocations with cgraph/alloc.h, but more
importantly reduces the amount of code to maintain here. Note that confusingly
the list begins with a NULL entry and is only relevant to store later if we have
accrued more than just the initial NULL.

This is the equivalent of b10b254860b2bcbc85b9779a2388d33ce8318908 but applied
to fdpgen.