cmd/link/internal/ppc64: Use PCrel relocs in runtime.addmoduledata if supported

cmd/link/internal/ppc64: Use PCrel relocs in runtime.addmoduledata if supported

This is another step towards supporting TOC-free operations.

Change-Id: I77edcf066c757b8ec815c701d7f6d72cd645eca9
Reviewed-on: https://go-review.googlesource.com/c/go/+/483437
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Michael Pratt <mpratt@google.com>