Prepend arch variant include directories

Arch variant include directories should override existing include
directories, add prepend_variant to the struct tags.

Test: m -j checkbuild
Change-Id: I4a758b42c19481f9496880d29dffea7836f613c5
diff --git a/cc/compiler.go b/cc/compiler.go
index a122bbd..eb30767 100644
--- a/cc/compiler.go
+++ b/cc/compiler.go
@@ -69,11 +69,11 @@
 	// If possible, don't use this.  If adding paths from the current directory use
 	// local_include_dirs, if adding paths from other modules use export_include_dirs in
 	// that module.
-	Include_dirs []string `android:"arch_variant"`
+	Include_dirs []string `android:"arch_variant,variant_prepend"`
 	// list of directories relative to the Blueprints file that will
 	// be added to the include path using -I
-	Local_include_dirs []string `android:"arch_variant"`
+	Local_include_dirs []string `android:"arch_variant,variant_prepend",`
 	// list of generated sources to compile. These are the names of gensrcs or
 	// genrule modules.