openbsd/arm does not support external linking - skip the note reading test that
uses linkmode external on this platform. While here, cleanup the code and
consistently use t.Skipf for all platforms that cannot run this test.

Change-Id: I64f0d9e038bc4c993c3d843fc069a0b723a924d6
Reviewed-on: https://go-review.googlesource.com/15054Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>