This changes the action graph when shared libraries are involved to always have
an action for the shared library (which does nothing when the shared library
is up to date).

Change-Id: Ibbc70fd01cbb3f4e8c0ef96e62a151002d446144
Reviewed-on: https://go-review.googlesource.com/8934Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>