cmd/asm,cmd/compile: generate preferred nop on PPC64

The preferred form of nop is ori 0,0,0. What was being generated was
or 0,0,0.

Fix a quirk in the assembler which effectively treats OR $0,Rx,Ry as
OR R0,Rx,Ry, and update the compiler to generate the preferred form.

Change-Id: I5ac4bf0258cff05b9eba516a767daebfc9e31bc7
Reviewed-on: https://go-review.googlesource.com/c/go/+/388974


Reviewed-by: Cherry Mui <cherryyz@google.com>
Trust: Paul Murphy <murp@ibm.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>