internal/bytealg: add PCALIGN to indexbodyp9 function on ppc64x

internal/bytealg: add PCALIGN to indexbodyp9 function on ppc64x

Adding PCALIGN in indexbodyp9 function shows
improvements in some SimonWaldherr benchmarks and one of the index
benchmarks on both Power9 and Power10

name              old time/op  new time/op  delta
Contains          19.8ns ± 0%  15.6ns ± 0%  -21.24%
ContainsNot       21.3ns ± 0%  18.9ns ± 0%  -11.03%
ContainsBytes     19.1ns ± 0%  16.0ns ± 0%  -16.54%
Index/10     17.3ns ± 0%    16.1ns ± 0%  -7.30%
Index/32     59.6ns ± 0%    59.6ns ± 0%  +0.12%
Index/4K     3.68µs ± 0%    3.68µs ± 0%    ~
Index/4M     3.74ms ± 0%    3.74ms ± 0%  -0.00%
Index/64M    59.8ms ± 0%    59.8ms ± 0%    ~

Change-Id: I784e57e0b0f5bac143f57f3a32845219e43d47fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/447595
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>