CB-6732 [WP8] Fix "MSBuild 64 bit is not supported" build error

https://issues.apache.org/jira/browse/CB-6732

Added fix based on process bitness check and exciting appropriate query. Tested for running build command from cli and directly calling build.bat from x64 and x86 modes. More details could be found here: http://msdn.microsoft.com/en-us/library/aa384274.aspx

Alternatively,  we can simply remove 'amd64' from the tools path returned by RegRead query