[Ada] fix lib and tools build

[Richard Kenner]

    Richard, I didn't follow closely the changes to the backend, are
    there any impossibility in getting the HEAD ada sources compilable
    in the 3.1 branch, I noticed nearly/all of your backend
    fixes have been commited to both head and branch.

I am testing using the ACT GNAT tree and the GCC 3.1 tree.  There is still
currently a lag between the ACT GNAT tree and the one at the FSF, though
I understand procedures are being developed to reduce that lag.

One issue that's going to be a pain in the upcoming months is the lang hook
changes, which will mean that GNAT for 3.1 and HEAD need to be different.
We'll have to figure out some procedure for making that as painless for
evertbody as possible.