Make react-native-codegen work on Windows within BUCK

Summary:
Changelog
[General][Internal] - Make react-native-codegen work on Windows within BUCK

This makes react-native code-gen work on Windows - by basically filling in some necessary windows commands for the existing bash/shell ones

Reviewed By: cortinico, cipolleschi

Differential Revision: D40268750

fbshipit-source-id: 0db7597999ec8108babf56abed436f6dc7789cce

packages/react-native-codegen/DEFS.bzl

@@ -536,13 +536,15 @@ def rn_codegen_cxx_modules(
     fb_native.genrule(
         name = generate_module_h_name,
         cmd = "cp $(location :{})/{}JSI.h $OUT".format(generate_fixtures_rule_name, name),
+        cmd_exe = "copy $(location :{})\\{}JSI.h $OUT".format(generate_fixtures_rule_name, name),
         out = "{}JSI.h".format(name),
         labels = ["codegen_rule"],
     )
 
     fb_native.genrule(
         name = generate_module_cpp_name,
         cmd = "cp $(location :{})/{}JSI-generated.cpp $OUT".format(generate_fixtures_rule_name, name),
+        cmd_exe = "copy $(location :{})\\{}JSI-generated.cpp $OUT".format(generate_fixtures_rule_name, name),
         out = "{}JSI-generated.cpp".format(name),
         labels = ["codegen_rule"],
     )