RFC/Ada: removal of $(ADAC) support

[Robert Dewar]

> No. There is only one C source file that needs to be compiled
> with the compiler that is used for the Ada source code: the one that
> defines __gnat_raise_exception.  Which is what the patch you rejected
> did.

That's just with respect to one particular problem. The connection between
the Ada parts of the code and the C parts of the code is intimate and
extensive, and even the slightest change in layout can disturb this connection
as well as other things. Why encourage people to try compiling GNAT with
non-matching compilers when this is known to be a potential source of
problems and is completely unnecessary.