cmd/compile/internal/ssa: include "ppc64" in has-regabi arches list

cmd/compile/internal/ssa: include "ppc64" in has-regabi arches list

While here, rename helper function to follow Go naming idioms:
https://github.com/golang/go/wiki/CodeReviewComments#initialisms

Fixes #53456.

Change-Id: I041dd25968e192ea2b430d58e3348626a970f92b
Reviewed-on: https://go-review.googlesource.com/c/go/+/422957
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>