cmd/internal/obj/ppc64: simplify huge frame prologue

CL 307010 for ppc64.

I spent a long time trying to figure out how to use the carry bit from
ADDCCC to further simplify this (like what we do on arm64), but gave
up after I couldn't figure out how to access the carry bit without
just adding more instructions.

Change-Id: I6cad51b93616865b203cb16554f16121375aabbc
Reviewed-on: https://go-review.googlesource.com/c/go/+/307149


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>