cmd/internal/asm/ppc64: avoid generating exser nops

cmd/internal/asm/ppc64: avoid generating exser nops

"OR $0, R31, R31" is the execution serializing nop called "exser"
on ISA 3.1 processors such as Power10.

In general, the "OR $0, Rx, Rx" where Rx != 0 form should be avoided
unless used explicitly for the uarch side-effects.

Change-Id: Id76e3a703c902676ba4a3ffb64dd90dad9a320bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/537855
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>