Add support for defaults modules

Add a cc_defaults module, which will prepend any matching properties to
modules that reference it with a "defaults" property.

Change-Id: I5908dd98d204f71b29ad95a4ab85403aa1621ca2
diff --git a/Android.bp b/Android.bp
index abae680..c887247 100644
--- a/Android.bp
+++ b/Android.bp
@@ -91,6 +91,7 @@
     srcs: [
         "common/arch.go",
         "common/config.go",
+        "common/defaults.go",
         "common/defs.go",
         "common/env.go",
         "common/glob.go",