s390x: Enable object backend (#4680)

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.

cranelift/codegen/src/isa/s390x/inst/mod.rs

@@ -3077,6 +3077,7 @@ impl MachInstLabelUse for LabelUse {
     fn from_reloc(reloc: Reloc, addend: Addend) -> Option<Self> {
         match (reloc, addend) {
             (Reloc::S390xPCRel32Dbl, 2) => Some(LabelUse::PCRel32Dbl),
+            (Reloc::S390xPLTRel32Dbl, 2) => Some(LabelUse::PCRel32Dbl),
             _ => None,
         }
     }