This cl allows SwiftShader to be built on Windows within Chromium

Change-Id: I0fc9ec1dfc5012cc310695e13edc64c639b23fda
Reviewed-on: https://swiftshader-review.googlesource.com/5680Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>