cmd/link: fix glink resolver generation on ppc64le

cmd/link: fix glink resolver generation on ppc64le

AddSymRef grows the section to fit the relocation. This was not
being accounted for. Instead, add a relocation and explicitly
populate it so we patch the desired instructions.

Change-Id: I583147e2545aea34c854f9d35ca920c57be60b90
Reviewed-on: https://go-review.googlesource.com/c/go/+/300949
Run-TryBot: Paul Murphy <murp@ibm.com>
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>