This enables the object backend for s390x, in particular the
processing of all required relocations.
This uncovered a bug: we need to use PLT relocations for the
target of calls, which we currently do not. Fixed by adding
a new S390xPLTRel32Dbl reloc type and using it where needed.
* Remove unused srcloc in MachReloc
* Remove unused srcloc in MachTrap
* Use `into_iter` on array in bench code to suppress a warning
* Remove unused srcloc in MachCallSite