cmd/internal/obj/ppc64: fix incorrect base reg causing segv

cmd/internal/obj/ppc64: fix incorrect base reg causing segv

This fixes a segv that was reported due to building minio. The
problem occurred because of an incorrect selection of the
base register, which was introduced by CL 306369.

Fixes #59196

Change-Id: Ieb77b2afa8fb4e6f3943df5ce138679f6750d376
Reviewed-on: https://go-review.googlesource.com/c/go/+/478920
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Archana Ravindar <aravind5@in.ibm.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Auto-Submit: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Paul Murphy <murp@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>