i686 ---> i586, i386 ---> i586, x86_64 stays the same

[Bill Nottingham]

Jakub Jelinek (jakub at redhat.com) said: 
> > Actually, in some testing I did, the fasted was '-march=i686
> > -mtune=generic' (which was ~1% faster than '-march=i586 -mtune=generic'),
> > tested across Core2Duo, Athlon64, and Atom.
> > 
> > Twiddling the instruction scheduling (-mtune) invariably made things
> > worse, across the board.
> 
> Even with -mtune=atom on Atom CPUs?

At the time I tested (early February), I don't think there was a
-mtune=atom.

Bill