powerpc64le: Require at least POWER8 for powerpc64le

Support for powerpc64le requires POWER8 or newer processors.  Builds for
older processors are not tested.  Require at least POWER8 to avoid
unintentional builds.

	* sysdeps/powerpc/powerpc64le/configure.ac: Check for POWER8.
	* sysdeps/powerpc/powerpc64le/configure: Update.