cmd/link: update plt size appropriately on ppc64

When attempting to enable internal linking with cgo on ppc64
it was discovered that the plt size was not being updated
after adding entries to it, which resulted in this error:

  .plt: initialize bounds (16 < 24)

This changes fixes that problem.

Updates #21961

Change-Id: Ie17539c329f5a4802e5defd93852dcdde19ded8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/261837


Trust: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>