[KNIFE-338] Fix bootstrap of 64-bit 2008r2 through FreeSSHd

http://tickets.opscode.com/browse/KNIFE-338

When connecting to FreeSSHd on 20008r2 you get a 32-bit environment. In this environment the PROCESSOR_ARCHITECTURE is reported as i686. This leads to the generation of an invalid URL for downloading the Chef installer.  The workaround is to first check PROCESSOR_ARCHITEW6432 which will show AMD64 even in a 32-bit environment.