Identify os architecture on windows 2003, 2008, 2012

Previous tests relied on wmi os osarchitecture property, which is not
present on Windows 2003.  Now we check osarchitecture first, and use
its result if successful.  If that attempt fails we fall back to
grepping the wmi os name property for an x64 string.

Sorry for the second patch.  The previous PR #47 did work for 2008, but we discovered that Windows 2003 lacks osarchitecture property.  Josh and I have tested this patch on 2003, 2008, 2008 R2, 2012 and 2012 R2 and it provisions using the correct version of cygwin for the platform arch.