add tests for LC_LOAD_DYLIB.

Change-Id: Ic4b7a0f6296709175e9a75240aecd1d5291ade4b
Reviewed-on: https://go-review.googlesource.com/56311Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>