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

[Paolo Bonzini]

> There is a Standard'Default_Bit_Order so it's the same as Word_Size: we just
> loose "source" documentation (and gain less diff between target file).

Yes, but Arnaud said that system-* constants are written down for a 
reason.  I don't understand *what* is the reason, but that's just 
because I have no clue (among other things) of what parts of the Ada RTS 
are installed.

For example, are these system-*.ads files installed?  If so, is it 
possible to install different versions for each multilib?  If not 
(question to Arnaud), is this self-documentation meant only for the 
people reading the source code of GNAT?  IOW, is it meant as a 
quick-and-dirty reference as to what are the characteristics of the target?

My impression is that 50% of the constants system-*.ads files are 
boilerplate (same for every file), 30% is easily derivable from 
configure.ac tests or from other Standard'xyz constants, and only 20% is 
actually something that depends on how GNAT works on the target.

If it was up to me, I would make the system.ads file automatically 
generated (except for the latter 20%, which would have to be specified 
in some way).  That would simplify multilibbing, but is moot unless 
there is a guarantee that this 20% is *totally* derivable from the 
target triplet, so that no conceivable flag combination can affect it. 
If this guarantee is not there, any attempt to multilib the Ada RTS is 
going to be a sore failure.

Paolo