internal/bytealg: add power9 version of bytes index

This adds a power9 version of the bytes.Index function
for little endian.

Here is the improvement on power9 for some of the Index
benchmarks:

Index/10           -0.14%
Index/32           -3.19%
Index/4K          -12.66%
Index/4M          -13.34%
Index/64M         -13.17%
Count/10           -0.59%
Count/32           -2.88%
Count/4K          -12.63%
Count/4M          -13.35%
Count/64M         -13.17%
IndexHard1        -23.03%
IndexHard2        -13.01%
IndexHard3        -22.12%
IndexHard4         +0.16%
CountHard1        -23.02%
CountHard2        -13.01%
CountHard3        -22.12%
IndexPeriodic/IndexPeriodic2  -22.85%
IndexPeriodic/IndexPeriodic4  -23.15%

Change-Id: Id72353e2771eba2efbb1544d5f0be65f8a9f0433
Reviewed-on: https://go-review.googlesource.com/c/go/+/311380


Run-TryBot: Carlos Eduardo Seo <carlos.seo@linaro.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Carlos Eduardo Seo <carlos.seo@linaro.org>
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>