bytes: add asm implementation for index on ppc64x

This adds an asm implementation of index on ppc64le and
ppc64. It results in a significant improvement in some
of the benchmarks that use bytes.Index.

The implementation is based on a port of the s390x asm
implementation. Comments on the design are found
with the code.

The following improvements occurred on power8:

Index/10       70.7ns ± 0%    18.8ns ± 0%   -73.4
Index/32        165ns ± 0%      95ns ± 0%   -42.6
Index/4K       9.23µs ± 0%    4.91µs ± 0%   -46
Index/4M       9.52ms ± 0%    5.10ms ± 0%   -46.4
Index/64M       155ms ± 0%      85ms ± 0%   -45.1

Count/10       83.0ns ± 0%    32.1ns ± 0%   -61.3
Count/32        178ns ± 0%     109ns ± 0%   -38.8
Count/4K       9.24µs ± 0%    4.93µs ± 0%   -46
Count/4M       9.52ms ± 0%    5.10ms ± 0%   -46.4
Count/64M       155ms ± 0%      85ms ± 0%   -45.1

IndexHard1     2.36ms ± 0%    0.13ms ± 0%   -94.4
IndexHard2     2.36ms ± 0%    1.28ms ± 0%   -45.8
IndexHard3     2.36ms ± 0%    1.19ms ± 0%   -49.4
IndexHard4     2.36ms ± 0%    2.35ms ± 0%    -0.1

CountHard1     2.36ms ± 0%    0.13ms ± 0%   -94.4
CountHard2     2.36ms ± 0%    1.28ms ± 0%   -45.8
CountHard3     2.36ms ± 0%    1.19ms ± 0%   -49.4

IndexPeriodic/IndexPeriodic2  146µs ± 0%       8µs ± 0%   -94
IndexPeriodic/IndexPeriodic4  146µs ± 0%       8µs ± 0%   -94

Change-Id: I7dd2bb7e278726e27f51825ca8b2f8317d460e60
Reviewed-on: https://go-review.googlesource.com/c/go/+/309730


Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Paul Murphy <murp@ibm.com>
Reviewed-by: Carlos Eduardo Seo <carlos.seo@linaro.org>
Trust: Carlos Eduardo Seo <carlos.seo@linaro.org>
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>