[Ada] multilib first patch

[Paolo Bonzini]

> To build and be later able to install more than one version of the ada
> library we need to change (at least) this assumption in some way
> and keep more than one library build result around.

The best way would be to bite the bullet, and move the RTS for real to 
libada (instead of having libada as a proxy).  The fact that the 
compiler needs it is not a problem, you just need to make gnat depend on 
a host libada.

However, this is not a small change.

Paolo