internal/bytealg: port bytes.Index and bytes.Count to reg ABI on ppc64x

internal/bytealg: port bytes.Index and bytes.Count to reg ABI on ppc64x

This change adds support for the reg ABI to the Index and Count
functions for ppc64/ppc64le.

Most Index and Count benchmarks show improvement in performance on
POWER9 with this change. Similar numbers observed on POWER8 and POWER10.

name                             old time/op    new time/op    delta
Index/32                         71.0ns ± 0%    67.9ns ± 0%   -4.42% (p=0.001 n=7+6)
IndexEasy/10                     17.5ns ± 0%    17.2ns ± 0%   -1.30% (p=0.001 n=7+7)

name             old time/op    new time/op    delta
Count/10           26.6ns ± 0%    25.0ns ± 1%   -6.02%  (p=0.001 n=7+7)
Count/32           78.6ns ± 0%    74.7ns ± 0%   -4.97%  (p=0.001 n=7+7)
Count/4K           5.03µs ± 0%    5.03µs ± 0%   -0.07%  (p=0.000 n=6+7)
CountEasy/10       26.9ns ± 0%    25.2ns ± 1%   -6.31%  (p=0.001 n=7+7)
CountSingle/32     11.8ns ± 0%     9.9ns ± 0%  -15.70%  (p=0.002 n=6+6)

Change-Id: Ibd146c04f8107291c55f9e6100b8264dfccc41ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/355509
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>