cmd/internal: Add 6 args to ppc64 optab

This is a preparatory patch to support 6 arg opcodes on POWER10,
and simplify 5 arg opcode processing (e.g RLWNM and similar).

This expands the optab structure, and renames a4 arguments to a6.
No actual change in functionality is made.

Change-Id: I785e4177778e4bf1326cf8e46e8aeaaa0e4d406b
Reviewed-on: https://go-review.googlesource.com/c/go/+/295031


Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Carlos Eduardo Seo <carlos.seo@linaro.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Trust: Keith Randall <khr@golang.org>