os: fix hex exit code print on 32-bit windows

We want to print hex exit codes for the large values,
but on 32-bit Windows the large values are negative.

Change-Id: I0e350b128414a9468c93eddc62d660f552c1ee05
Reviewed-on: https://go-review.googlesource.com/c/go/+/294070


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>