Specify target platform for MSBUILD

MSBUILD defaults to platform X64 if none is specified on the command line
on a 64bit machine. If the configuration in the solution does not exist,
MSBUILD will fail.

The is the output on a 64bit machine.
e:\dev\gitrepos\ironpython3\IronPython.sln.metaproj : error MSB4126: The specified solution configuration "v2Debug|X64" is
invalid. Please specify a valid solution configuration using the Configuration and Platform properties (e.g. MSBuild.exe So lution.sln /p:Configuration=Debug /p:Platform="Any CPU") or leave those properties blank to use the default solution config uration. [E:\Dev\gitrepos\ironpython3\IronPython.sln]