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

[Arnaud Charlet]

>> Right, that's one possibility, although people seem to be focusing
>> in system.ads alot, which is actually only the tip of the iceberg, and not
>> a real issue per se.
> 
> Still it's the biggest problem so far.

No, the issue is setting the proper target when building gnatlib, most things
will fall out for free once this is solved.

Arno