[dev.ssa] cmd/compile: PPC64, add more zeroing and moves

[dev.ssa] cmd/compile: PPC64, add more zeroing and moves

Passes light testing.
Modified to avoid possible exposure of "exterior" pointers
to GC.

Updates #16010.

Change-Id: I41fced4fa83cefb9542dff8c8dee1a0c48056b3c
Reviewed-on: https://go-review.googlesource.com/25310
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>