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/emit.rs

@@ -3216,7 +3216,7 @@ impl MachInstEmit for Inst {
                 }
 
                 let opcode = 0xc05; // BRASL
-                let reloc = Reloc::S390xPCRel32Dbl;
+                let reloc = Reloc::S390xPLTRel32Dbl;
                 if let Some(s) = state.take_stack_map() {
                     sink.add_stack_map(StackMapExtent::UpcomingBytes(6), s);
                 }