cmd/link,cmd/internal/obj/ppc64: enable PCrel on power10/ppc64/linux

cmd/link,cmd/internal/obj/ppc64: enable PCrel on power10/ppc64/linux

A CI machine has been set up to verify GOPPC64=power10 on ppc64/linux.

This should be sufficient to verify the PCrel relocation support works
for BE.

Note, power10/ppc64/linux is an oddball case. Today, it can only link
internally. Furthermore, all PCrel relocs are resolved at link time,
so it works despite ELFv1 having no official support for PCrel relocs
today.

Change-Id: Ibf79df69406ec6f9352c9d7d941ad946dba74e73
Reviewed-on: https://go-review.googlesource.com/c/go/+/485075
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Bryan Mills <bcmills@google.com>