[ELF] Make createThunks a class [NFC]

For range extension thunks we will need to repeatedly call createThunks()
until no more thunks are created. We will need to retain the state of
Thunks that we have created so far to avoid recreating them on later
passes. This change does not change the functionality of createThunks().

Differential Revision: https://reviews.llvm.org/D31654