cmd,cmd/vendor: pick up updates for golang.org/x/arch/ppc64

Bring in updates to golang.org/x/arch/ppc64 to add new
instructions from CLs 260617, 230957, 249158.

Used the directions found in README.vendor:

  cd $GOROOT/src/cmd
  go get -d golang.org/x/arch@latest
  go mod tidy
  go mod vendor

Change-Id: Ie8ceb9c804928b72b37fff5b63981c174fb86989
Reviewed-on: https://go-review.googlesource.com/c/go/+/264079


Trust: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>