RFC/Ada: removal of $(ADAC) support

[Richard Kenner]

    I reiterate that there is no known problem in this domain other than
    the one affecting __gnat_raise_exception; you are spreading FUD.

You are doing exactly the same thing by claiming that "gnatgcc" won't
successfully bootstrap the compiler.

There are two possible sources of problems:

(1) The "gnatgcc" may only compile Ada correctly and not C.
(2) There may be a layout difference between "gnatgcc" and "gcc" in
    some structure.

We have zero evidence for *either* of these things happening and I'd argue
that neither is likely.

That argues for the simpler approach, which is to use the same compiler
on all sources.