Ada front-end depends on signed overflow

[Paul Schlie]

> (Without -fwrapv, integer overflow is undefined, and subsequent range
> checks can be optimized away, so that it might cause erroneous
> behavior.)

- Since for all practical purposes most (if not all) target's use
  2's complement integer representations which naturally "wrap", might
  it be simply best to presume that all do "wrap" by default, but allow
  -fnowrapv to disable it if ever required by the odd target/language?