[RFC] Add aarch64 support for ada

[Arnaud Charlet]

The Makfile.in and init.c changes are OK.

The types.h change is likely more controversial and may be problematic,
I'll let Eric comment.

> +  system.ads<system-linux-x86_64.ads
> 
> IMO, this should really be called system-linux-lp64.ads, and should
> be usable for any 64-bit target that uses full ieee floating point,
> which is all of them.

Well, in our experience, each time we've tried to share system files,
this came back and bit us at some point. But I do not know the
aarch64 architecture to comment on this specific case.

Arno