The definition of writeBarrier in the runtime was changed in CL 22855
to include padding. Update the definition built in to the compiler to match.
This doesn't affect the generated code, as the compiler sets the type
to use anyhow, but having them be different seems clearly wrong.

Change-Id: I8eac05bf70a424a0b2338ba5e9e41af231316de0
Reviewed-on: https://go-review.googlesource.com/37377
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>