Allow "required" property to vary by arch

Renderscript needs to require different bitcode files based
on architecture.

Test: builds
Change-Id: Ifffa16c892a7f8e309f1c5652472fba44cd2e786
diff --git a/android/module.go b/android/module.go
index a38c617..90d61e2 100644
--- a/android/module.go
+++ b/android/module.go
@@ -154,7 +154,7 @@
 	Init_rc []string
 
 	// names of other modules to install if this module is installed
-	Required []string
+	Required []string `android:"arch_variant"`
 
 	// Set by TargetMutator
 	CompileTarget  Target `blueprint:"mutated"`