s390x: Basic support for IaddIfcout

This adds enough support for the IaddIfcout opcode to make the
code emitted by dynamic_addr work on s390x.

Note: On s390x, the condition code mask that has to be used to
implement unsigned_add_overflow_condition does not match any of
the masks for the "normal" condition codes, so this design is
not really a good match for s390x ...

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

@@ -121,7 +121,12 @@ impl MachBackend for S390xBackend {
     }
 
     fn unsigned_add_overflow_condition(&self) -> IntCC {
-        unimplemented!()
+        // The ADD LOGICAL family of instructions set the condition code
+        // differently from normal comparisons, in a way that cannot be
+        // represented by any of the standard IntCC values.  So we use a
+        // dummy value here, which gets remapped to the correct condition
+        // code mask during lowering.
+        IntCC::UnsignedGreaterThan
     }
 
     fn unsigned_sub_overflow_condition(&self) -> IntCC {