cmd/internal: cleanup ppc64 optab structure

This is no-functionality change to begin the process of supporting
more than 6 operands.

This rewrites the table to use named arguments, and removes default
initialized argument values.  The following sed regexes rewrote the table:

  s/{\([^,]*\),\([^,]*\),\([^,]*\),\([^,]*\),\([^,]*\),\([^,]*\),\([^,]*\),\([^,]*\)}/{as:\1,a1:\2,a2:\3,a3:\4,a4:\5,type_:\6,size:\7,param:\8}
  s/a[1-4]: C_NONE, //g
  s/, param: 0//

Change-Id: I5f4de9da75f2fb3964d625d6b4e2f1ce1e29cc47
Reviewed-on: https://go-review.googlesource.com/c/go/+/294189
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: Carlos Eduardo Seo <carlos.seo@linaro.org>