This uses weak declarations so that it will work with current versions
of gccgo that do not support pointer checking.

Change-Id: Ia34507e3231ac60517cb6834f0b673764715a256
Reviewed-on: https://go-review.googlesource.com/17429
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>