[Ada] multilib patch / -imultilib and language drivers

[Paolo Bonzini]

> What triggers the passing of -imultilib to a language driver?

It is passed together with -isystem, -iprefix and friends when %I is in 
a spec.  I'm sure you can define a new spec function and use it to pass 
the multilib_dir variable down to the Ada driver (see default_compilers, 
I guess you have to read some gcc.c).

> Once gcc passes this info to gnat1 it will likely be easy to have
> gnatmake/bind/link extract it when needed since those tools
> call gcc.

I believe you on this. :-)

Paolo