[PATCH][4.3] Deprecate -ftrapv

[Paolo Bonzini]

> Seems a pity, this feature is really needed for Ada, which requires
> efficient overflow checking for signed arithmetic, and it seems
> useful for C, where signed overflow is undefined, and it would be
> nice to enable trapping.
> 
> Right now, Ada uses really horrible double length arithmetic to
> do trapping.

It's not the only way to do it though, and it's best to reimplement 
-ftrapv in terms of GIMPLE.

I'm for deprecating -ftrapv and reimplementing it.  I could mentor a 
GSOC project for reimplementation of -ftrapv.

Paolo