os: align siginfo argument to waitid

Backport https://golang.org/cl/46511 from gc trunk, as it may fix a
bug reported for gccgo running on MIPS
(https://groups.google.com/d/msg/golang-dev/sDg-t1_DPw0/-AJmLxgPBQAJ).

Change-Id: Icff4c70cc469b3742a03fe19a21f840dae8bfd38
Reviewed-on: https://go-review.googlesource.com/46571
Reviewed-by: Ian Lance Taylor <iant@golang.org>
1 file changed