cmd/internal/obj/ppc64: add SETB instruction

cmd/internal/obj/ppc64: add SETB instruction

This ISA 3.0 (power9) instruction is helpful for some string functions
in a future change.

Change-Id: I1a659488ffb5099f8c89f480c39af4ef9c4b556a
Reviewed-on: https://go-review.googlesource.com/c/go/+/472635
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Archana Ravindar <aravind5@in.ibm.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>