Minimum Go requirement for ppc64(le) architecture support is POWER8.
https://github.com/golang/go/wiki/MinimumRequirements#ppc64-big-endian

Reduce CPU features supported in internal/cpu to those needed to
test minimum requirements and cpu feature kernel support for ppc64(le).
Currently no internal/cpu feature variables are used to guard code
from using unsupported instructions. The IsPower9 feature variable
and detection is kept as it will soon be used to guard code execution.

Reducing the set of detected CPU features for ppc64(le) makes
implementing Go support for new operating systems easier as
CPU feature detection for ppc64(le) needs operating system support
(e.g. hwcap on Linux and getsystemcfg syscall on AIX).

Change-Id: Ic4c17b31610970e481cd139c657da46507391d1d
Reviewed-on: https://go-review.googlesource.com/c/145117
Run-TryBot: Martin Möhrmann <martisch@uos.de>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Carlos Eduardo Seo <cseo@linux.vnet.ibm.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>