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/jit/src/compiled_blob.rs

@@ -72,7 +72,7 @@ impl CompiledBlob {
                         write_unaligned(at as *mut i32, pcrel)
                     };
                 }
-                Reloc::S390xPCRel32Dbl => {
+                Reloc::S390xPCRel32Dbl | Reloc::S390xPLTRel32Dbl => {
                     let base = get_address(name);
                     let what = unsafe { base.offset(isize::try_from(addend).unwrap()) };
                     let pcrel = i32::try_from(((what as isize) - (at as isize)) >> 1).unwrap();