This test requires external linking, but we don't yet implement
external linking on ppc64 (tracked in issue #8912).  Disable the test
on ppc64 until external linking is implemented.

This makes all.bash pass on ppc64le.

Change-Id: I741498d4d9321607e7a65792a33faf8187bd18e4
Reviewed-on: https://go-review.googlesource.com/2908Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>