[m68k] cleanup arch/cpu selection

[Nathan Sidwell]

Ben Elliston wrote:
>>This patch implements -march= and -mcpu= options to m68k's
>>assembler.  It also adds a set of -m[no-]foo extension switches.  In
>>addition I've rationalized the coldfire bfd_mach enumeration to work
>>along the lines of architectural features, rather than chip
>>numbering -- there's an ever increasing number of chips.  I've kept
>>the old machine names and old assembler switches for compatibility.
> 
> 
> This seems like a sensible proposal to me, however please ensure that
> for the time being, the old-style command line options continue to be
> supported.  If you like, you could emit a warning and advise the user
> of the new options.  I really dislike seeing users left to work out
> why their Makefiles have stopped working when they upgrade binutils.

This is the patch I've committed.  It addresses Andreas's concern about the EF_ 
flags. In doing so I discovered I'd missed a piece of readelf, which needed 
adjusting.

nathan
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