benchmark: fix child-process-exec-stdout on win

This benchmark fails on Windows when trying to execute command which
is more than 32k in size. This commits skips this one case when running
under Windows.

PR-URL: https://github.com/nodejs/node/pull/7178


Reviewed-By: Trott - Rich Trott <rtrott@gmail.com>
Reviewed-By: orangemocha - Alexis Campailla <orangemocha@nodejs.org>