Cleanups for the m68k backend

[Jim Wilson]

Bernardo Innocenti wrote:
>  Who is using m68020-*-elf*?

Probably no one.  Keep in mind that gcc isn't the only program that uses 
these configure names.  There is also binutils, gdb, newlib, etc. 
However, I don't see support for m68020-elf in all of them, and a quick 
uberbaum tree confirms this.  So m68020-elf isn't useful anymore and can 
be eliminated.

I think the point of having m68020-elf was so that we could have a 
target that defaults to m68020 code instead of one that defaults to 
m68000 code.  In current sources, a better way to do this is via 
--with-cpu configure options.

> Could we drop it and rename "m68020-elf.h"
> to "m68k-elf.h"

It would be very reasonable to do this.

Jim