cgo: fix declarations in _cgo_export.c
cgo: fix declarations in _cgo_export.c

Declare crosscall2.  Declare the functions passed to it as
returning void, rather than relying on implicit return type.

R=golang-dev, minux.ma
CC=golang-dev
http://codereview.appspot.com/6432060
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go