internal/bytealg: improve compare on Power10/PPC64

internal/bytealg: improve compare on Power10/PPC64

Handle comparisons of 15 or less bytes more efficiently
with Power10 instructions when building with GOPPC64=power10.

name            old time/op  new time/op  delta
BytesCompare/1  2.53ns ± 0%  2.17ns ± 0%  -14.17%
BytesCompare/2  2.70ns ± 0%  2.17ns ± 0%  -19.77%
BytesCompare/4  2.59ns ± 0%  2.17ns ± 0%  -16.20%
BytesCompare/8  2.66ns ± 0%  2.17ns ± 0%  -18.63%

Change-Id: I6d7c6af0a58ea3e03acc3930c54b77f2ac1dfbd5
Reviewed-on: https://go-review.googlesource.com/c/go/+/522315
Reviewed-by: Joedian Reid <joedian@golang.org>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>