cmd/internal/obj/ppc64: add VC[LT]ZLSBB instructions

cmd/internal/obj/ppc64: add VC[LT]ZLSBB instructions

These are ISA 3.0 power9 instructions which are helpful when reducing
a vector compare result into a GPR.

They are used in a future patch to improve the bytes.IndexByte asm
routine.

Change-Id: I424e2628e577167b9b7c0fcbd82099daf568ea35
Reviewed-on: https://go-review.googlesource.com/c/go/+/478115
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>