use init functions to register module types, etc.

Instead of putting all the blueprint registrations in soong_build,
put them all in init() functions.  This puts the registration next
to the implementation.

Change-Id: Ide1a749518f5e9d1367a18ab3bb1d91da3310c76
diff --git a/common/arch.go b/common/arch.go
index 6865904..c0e63eb 100644
--- a/common/arch.go
+++ b/common/arch.go
@@ -20,10 +20,17 @@
 	"runtime"
 	"strings"
 
+	"android/soong"
+
 	"github.com/google/blueprint"
 	"github.com/google/blueprint/proptools"
 )
 
+func init() {
+	soong.RegisterEarlyMutator("host_or_device", HostOrDeviceMutator)
+	soong.RegisterEarlyMutator("arch", ArchMutator)
+}
+
 var (
 	Arm    = newArch("arm", "lib32")
 	Arm64  = newArch("arm64", "lib64")