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

[Paolo Bonzini]

> This will need some additionals tests on MULTILIB in the LIBGNAT_TARGET_PAIRS
> machinery (3 files for x86 vs x86_64, solaris looks like already done, powerpc
> seem 32 bits only right now, s390/s390x, others?) but it doesn't seem like a
> blocking issue with the proposed design since each MULTILIB rts build has
> completely separate directory and stamp (through RTSDIR) so there is no
> possibility of conflict through sharing.
> 
> Do you agree with this assessment?

Unfortunately not.  The solaris bits are just "factoring" a bit the 
definition of LIBGNAT_TARGET_PAIRS.  The actual multilib definition can 
be arbitrary, for example you can have big-endian/little-endian 
multilibs.  If the design is explicitly to have constants spelled out in 
system-* files (instead of having them, for example, in Autoconf 
macros), there is not much that you can do.

Paolo