Support cross-compiling Windows binaries on Linux

This defines another mutator between HostOrDevice and Arch that will
expand host modules into a module for each host type
(Darwin/Linux/Windows) that is currently being built.

Change-Id: I4c8ac6b616c229f6bd45ad8a35902652fb6a4fff
diff --git a/cc/arm_device.go b/cc/arm_device.go
index 746de2e..2d6d38c 100644
--- a/cc/arm_device.go
+++ b/cc/arm_device.go
@@ -362,5 +362,5 @@
 }
 
 func init() {
-	registerToolchainFactory(common.Device, common.Arm, armToolchainFactory)
+	registerDeviceToolchainFactory(common.Arm, armToolchainFactory)
 }