Only check WOW64 process when system architecture is x64.

Fixes https://github.com/opscode/chef/issues/1549.

The issue here is that according to [msdn](http://msdn.microsoft.com/en-us/library/windows/desktop/ms684139(v=vs.85).aspx) IsWow64Process returns `false` on 32 bit windows systems. Since we're not differentiating 32 bit systems from 64 bit systems we're raising an unnecessary error here.

Windows specs in CI is happy with this [here](http://ci.opscode.us/job/chef-client-test/828/).

I'm open to other solutions @jmink, @btm :smile: