Starting in go1.9, the minimum processor requirement for ppc64 is POWER8.
Therefore, the checks for OldArch and the code enabled by it are not necessary
anymore.

Updates #19074

Change-Id: I33d6a78b2462c80d57c5dbcba2e13424630afab4
Reviewed-on: https://go-review.googlesource.com/38404
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>