Make sure that the same type and itab generated in two
different shared library are actually the same thing.

Change-Id: Ica45862d65ff8bc7ad04d59a41f57223f71224cd
Reviewed-on: https://go-review.googlesource.com/35115
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>