cmd/asm,cmd/internal/obj/ppc64: add {l,st}xvx power9 instructions

These are the indexed vsx load operations with the
same endian and alignment benefits of {l,st}vx.

Likewise, cleanup redundant comments in op{load,store}x and
fix ISA 3.0 typos nearby.

Change-Id: Ie1ace17c6150cf9168a834e435114028ff6eb07c
Reviewed-on: https://go-review.googlesource.com/c/go/+/249025


Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>