Nonrecursive build within gcc directory

[Richard Kenner]

    This patch rearranges the Makefiles so that we don't need to recurse
    into the language subdirectories.  Basically all the important logic
    from each language Makefile.in is moved to the corresponding
    Make-lang.in, and all the paths are adjusted to work from the top
    level, rather than the language subdir.  We can then throw away the
    Makefile.in.  I think this arrangement is both conceptually simpler,
    and cleaner in implementation.

This still *allows* a language to have Makefile.in, right?

For Ada, the two files are *huge*, with Makefile.in changing nearly
every day.  Having to maintain two very different styles of files during
the conversion between GCC versions will be *a lot* of work.