Updating the templates to create win64 projects.

templates/vsprojects/grpc.sln.template

templates/vsprojects/vcxproj_defs.include

vsprojects/gpr/gpr.vcxproj

vsprojects/gpr_test_util/gpr_test_util.vcxproj

vsprojects/grpc++/grpc++.vcxproj

vsprojects/grpc.sln

vsprojects/grpc/grpc.vcxproj

vsprojects/grpc/grpc.vcxproj.filters

vsprojects/grpc_csharp_ext/grpc_csharp_ext.vcxproj

vsprojects/grpc_test_util/grpc_test_util.vcxproj

...s/grpc_test_util_unsecure/grpc_test_util_unsecure.vcxproj

vsprojects/grpc_unsecure/grpc_unsecure.vcxproj

vsprojects/grpc_unsecure/grpc_unsecure.vcxproj.filters