misc/cgo: fix issue 9400 test on 386

issue9400_linux.go did not build on 386 because it used a constant
that was larger than a 32-bit int in a ... argument.  Fix this by
casting the constant to uint64 (to match how the constant is being
used).

Change-Id: Ie8cb64c3910382a41c7852be7734a62f0b2d5a21
Reviewed-on: https://go-review.googlesource.com/2060
Reviewed-by: Ian Lance Taylor <iant@golang.org>