…ack_noctxt on ppc64

In the fix for 54332 the MOVD R1, R1 instruction was added to
morestack_noctxt function to set the SPWRITE bit. However, the
instruction MOVD R1, R1 results in or r1,r1,r1 which is a special
instruction on ppc64 architecture as it changes the thread priority
and can negatively impact performance in some cases.
More details on such similar nops can be found in Power ISA v3.1
Book II on Power ISA Virtual Environment architecture in the chapter
on Program Priority Registers and Or instructions.
Replacing this by OR R0, R1 has the same affect on setting SPWRITE as
needed by the first fix but does not affect thread priority and
hence does not cause the degradation in performance

Hash65536-64           2.81GB/s ±10%  16.69GB/s ± 0%  +494.44%
Fixes #57812

Change-Id: Ib912e3716c6afd277994d6c1c5b2891f82225d50
Reviewed-on: https://go-review.googlesource.com/c/go/+/461597
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Auto-Submit: Benny Siegert <bsiegert@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
(cherry picked from commit 1c65b69)
Reviewed-on: https://go-review.googlesource.com/c/go/+/462335
Run-TryBot: Archana Ravindar <aravind5@in.ibm.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>