cmd/link: support R_ADDRPOWER_GOT_PCREL34 on ppc64le

cmd/link: support R_ADDRPOWER_GOT_PCREL34 on ppc64le

This will be generated by go code built for dynamic linking when
built with GOPPC64=power10 in a future change.

Change-Id: I9bd7ca6b2a65c360465e35ef12b10d3fa6d75745
Reviewed-on: https://go-review.googlesource.com/c/go/+/463015
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Archana Ravindar <aravind5@in.ibm.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>