cmd/link: add support for openbsd/ppc64

cmd/link: add support for openbsd/ppc64

Add linker support for the openbsd/ppc64 port.

Updates #56001

Change-Id: I18bc19b4086599996aebfbe68f2e85e1200589ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/475619
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Paul Murphy <murp@ibm.com>
Reviewed-by: Eric Grosse <grosse@gmail.com>