Gccgo isn't locking the OS thread properly during calls.

Change-Id: Idb2475291405e390cbb83abb27a402fd0381d0c4
Reviewed-on: https://go-review.googlesource.com/18882
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>