cmd/internal: cleanup ppc64 optab structure
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 <b27445fe6fb259c37b8e0a4ce56e22e2413754bf@linux.vnet.ibm.com>
Run-TryBot: Lynn Boger <b27445fe6fb259c37b8e0a4ce56e22e2413754bf@linux.vnet.ibm.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Carlos Eduardo Seo <7747efeb1dc5d836a1029889c6ce91c3a93b63dc@linaro.org>

CAFxX/go,golang/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,golang/go,cloudflare/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go