[PATCH] Document arithmetic overflow semantics

[Robert Dewar]

> The fact that the behaviour of overflow is undefined in C, C++ and Ada
> does not mean that "overflow cannot occur".  Quite clearly overflows do
> occur, and the optimizers are paralyzed by the semantics in this event.

Since this quote is being repeated many times, it is perhaps worth
emphasizing that overflow *is* defined in Ada, and indeed the most
important thing that Ada needs from gcc is efficient code for arithmetic
operations that trap on overflow :-)