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

[Paolo Bonzini]

> I think you will end up having to support generating different
> source trees for each multilib variant to be safe and correct.

Yes, that comes out naturally if the RTS is built in libada.  In fact, 
Arnaud said:

>>>> The idea currently is to make these values
>>>> explicit so that when people read system.ads, they know right away what
>>>> the right value is.

That's "when people read system.ads", not "when people read 
system-linux-x86.ads".  In other words, he's not necessarily against 
automatically generating system.ads from other means, for example using 
configure tests.  Which, I repeat, comes out naturally if the RTS build 
is confined in libada.

> This will work for native builds but may have problems on
> cross builds where you can't run a program.  I know for the
> RTEMS g-soccon* file we have to run a program on target
> hardware and capture the output.

Do you really need to run programs?  Most of gen-soccon can be done by 
running Ada source code through the C pre-processor and massaging the 
output.  In fact, the code that would be passed through cpp strongly 
resembles gen-soccon.c itself.

> If you move the source to libada and start potentially
> using different source combinations for different multilib
> variants, then it does need to be on a branch.
> 
> But some of the patches so far seem like they would be OK
> to commit on the mainline and minimize diffs.

Yes, that's true.

Paolo