RFC/Ada: removal of $(ADAC) support

[Richard Kenner]

    I don't know.  I believe that ARM and/or MIPS were included though.

I don't know of any ARM Ada host, so the only issue is MIPS, which is
not that common now.  So the only cost of having a common compiler for
front ends and backends is that a bootstrap from MIPS *might* not work
without a more recent compiler.  I think that's a small price to pay for
consistency: remember that if we use $(ADAC) for some C files, it complicates
the Makefiles.