Ada front-end depends on signed overflow

[Robert Dewar]

Geert Bosch wrote:

> For Ada, I propose we make the following changes:
>   - by default, enable overflow checks using -ftrapv

This won't work, we generate plenty of cases of operations
where we definitely don't want a check, and we don't set the
flag, but currently the flag is ignored by gcc/gigi, we would
have to adjust gigi to switch to unsigned for cases where the
check overflow flagwas not set

>     (jay, we should be able to get rid of -gnato finally!)
>   - with checks suppressed, use -fwrapv.

We really need to do this on an operation by operation basis,
since in Ada checks can be suppressed on an operation by
operation basis!