[Ada] Bootstrapping mainline GNAT fails

[Richard Kenner]

    Usually, people use the phrase "GNAT based on GCC 3.0" in a completely
    different sense: GNAT is said to be based on a particular GCC version
    if it *links* with the back end code (which is written in C) of that
    GCC version (and not if the back end code is compiled using that GCC
    version).

That's I meant by it.  The issue here was using a GCC 3.0-based GNAT to
compile the current sources.