cmd/link: support R_PPC64_REL24_P9NOTOC

cmd/link: support R_PPC64_REL24_P9NOTOC

This can be treated identically to R_PPC64_REL24_NOTOC as stubs
are generated based on GOPPC64 and -buildmode.

Change-Id: I3c42a9bf3c08d107fb656e6bf59b2307783bd9af
Reviewed-on: https://go-review.googlesource.com/c/go/+/497915
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>