runtime: improve ppc64x memclr for tail bytes

runtime: improve ppc64x memclr for tail bytes

This improves memclr for the last few bytes when
compiling for power9 or earlier.

Change-Id: I46940ebc7e98e27a2e48d4b319acb7d2106a6f29
Reviewed-on: https://go-review.googlesource.com/c/go/+/495035
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Paul Murphy <murp@ibm.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>