Fix s390x regressions (#3330)

- Add relocation handling needed after PR #3275
- Fix incorrect handling of signed constants detected by PR #3056 test
- Fix LabelUse max pos/neg ranges; fix overflow in buffers.rs
- Disable fuzzing tests that require pre-built v8 binaries
- Disable cranelift test that depends on i128
- Temporarily disable memory64 tests

cranelift/codegen/src/machinst/buffer.rs

@@ -597,7 +597,7 @@ impl<I: VCodeInst> MachBuffer<I> {
             self.island_worst_case_size += kind.veneer_size();
             self.island_worst_case_size &= !(I::LabelUse::ALIGN - 1);
         }
-        let deadline = offset + kind.max_pos_range();
+        let deadline = offset.saturating_add(kind.max_pos_range());
         if deadline < self.island_deadline {
             self.island_deadline = deadline;
         }