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

[Joseph Myers]

On Fri, 20 Oct 2017, 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.

There is a configure test that for powerpc64le the (compiler + options 
used) is building for POWER8 or later.  *But* this does not ensure that 
the power8 sysdeps directories are used.

The preferred approach for deciding which sub-architecture sysdeps 
directories to use is that used in sysdeps/arm/preconfigure.ac - examine 
what machine the compiler builds for.  However, powerpc is still using the 
older approach of requiring --with-cpu= options to select such sysdeps 
directories, and there is no requirement to use --with-cpu=power8 for 
powerpc64le.  (Some architectures use a third approach, of basing things 
on the host triplet.)

It would be a good idea to move powerpc to testing what the compiler does, 
and generally to obsolete --with-cpu= for all architectures.

-- 
Joseph S. Myers
joseph@codesourcery.com