cmd/compile/internal: generate ADDZE on PPC64

cmd/compile/internal: generate ADDZE on PPC64

This usage shows up in quite a few places, and helps reduce
register pressure in several complex cryto functions by
removing a MOVD $0,... instruction.

Change-Id: I9444ea8f9d19bfd68fb71ea8dc34e109681b3802
Reviewed-on: https://go-review.googlesource.com/c/go/+/571055
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>