obj.ARET is the portable return mnemonic. ppc64.ARETURN is a legacy
alias.

This was done with
    sed -i s/ppc64\.ARETURN/obj.ARET/ cmd/compile/**/*.go
    sed -i s/ARETURN/obj.ARET/ cmd/internal/obj/ppc64/obj9.go

Change-Id: I4d8e83ff411cee764774a40ef4c7c34dcbca4e43
Reviewed-on: https://go-review.googlesource.com/10673Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>