cmd/link: don't overwrite text sections on PPC64

The code writes text sections twice, one with Codeblk, one with
Datblk. The second write shouldn't be there.

May fix #38898.

Change-Id: I4ec70294059ec9aa0fc4cc69a3cd824f5843287b
Reviewed-on: https://go-review.googlesource.com/c/go/+/232661


Run-TryBot: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>