[Ada] Fix PR ada/51483

[Geert Bosch]

On Mar 13, 2014, at 11:36, Eric Botcazou <ebotcazou@adacore.com> wrote:

> This fixes a flaw in the mechanism implemented to register modes and types
> declared in the back-end with the front-end. The mechanism was implicitly
> making the assumption that it is possible to deduce the size of a FP mode
> from its precision and alignment; that's wrong in the general case, although
> exceptions to the rule are quite rare (typically for an IEEE extended mode).

Thanks, Eric!

  -Geert