linux-user: set ppc64/ppc64le default CPU to POWER8

linux-user: set ppc64/ppc64le default CPU to POWER8

Set the default to the latest CPU version to have the
largest set of available features.

It is also really needed in little-endian mode because
POWER7 is not really supported in this mode and some distros
(at least debian) generate POWER8 code for their ppc64le target.

Fixes: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=813698
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>