cmd/go/testdata/mod: add golang toolchain test data for openbsd/ppc64

Updates #56001

Change-Id: Ic7b4ecb2e471292894c54610e8acda8822c890fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/518275


Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Paul Murphy <murp@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>