[POWERPC] Don't override gcc -mcpu with wrong asm .machine

[Alan Modra]

On Fri, Oct 20, 2017 at 03:51:54PM -0200, Tulio Magno Quites Machado Filho wrote:
> I understand that ppc64le was never tested on POWER4, but I'm afraid this
> failure is actually showing there is another problem.

Yes, why is powerpc64/multiarch/memcmp-power4.S being compiled for
powerpc64le?  And even with --disable-multiarch!

-- 
Alan Modra
Australia Development Lab, IBM