[Ada] multilib patch take two => multilib build working

[laurent@guerby.net]

Selon Arnaud Charlet <charlet@adacore.com>:
> Well, also the issue is that currently the Makefile for building the run-time
> is not prepared to handle all the possible combinations currently supported
> by GCC on some platforms, so things will also break in various places if
> you enable multilibs blindly.

Right.

Paolo, do you know where to look for the list of multilibs on a given platform
in the GCC sources? And if we want to disable some of them for Ada?

Thanks in advance,

Laurent