Enabling Subzero compilation on Windows

Once Subzero is pulled by Chromium (see
https://codereview.chromium.org/2695813002/),
we'll enable Subzero compilation on Windows.

Change-Id: I3464dbd98224b3ac602aef3c4a27596366d275b3
Reviewed-on: https://swiftshader-review.googlesource.com/8772
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
diff --git a/src/Reactor/BUILD.gn b/src/Reactor/BUILD.gn
index ee93357..d74f06a 100644
--- a/src/Reactor/BUILD.gn
+++ b/src/Reactor/BUILD.gn
@@ -15,7 +15,7 @@
 declare_args() {
   # Currently, Subzero is not used by default
   # LLVM is still the default backend
-  use_swiftshader_with_subzero = false
+  use_swiftshader_with_subzero = is_win
 }
 
 # Need a separate config to ensure the warnings are added to the end.