janitor: Use `fi` and `di` from u_math.h across the tree

Doing grep -r -A10 union src/ | less, then searching for float will find a bunch of places throughout the tree that redefine data structures exactly like fi from u_math.h. Similar duplication exists for di. We could just use those existing types across the whole tree instead.

It may not be possible to make changes in some pieces of imported code (e.g., AMD's addrlib).