widevine: support multilib builds

Support builds for multiple architectures the same time:
Use LOCAL_MODULE_TARGET_ARCH to restrict building to specific arches
Use LOCAL_C_INCLUDES_x86 to set the include path for only x86 builds

Change-Id: I7c33c27f1c9bfb6e3318a07514698992482f6cd7

proprietary/streamcontrol/lib/x86/Android.mk

@@ -1,5 +1,3 @@
-ifeq ($(TARGET_ARCH),x86)
-
 LOCAL_PATH:= $(call my-dir)
 
 include $(CLEAR_VARS)
@@ -12,6 +10,5 @@ LOCAL_PROPRIETARY_MODULE := true
 LOCAL_STRIP_MODULE := true
 
 LOCAL_MODULE_TAGS := optional
+LOCAL_MODULE_TARGET_ARCH := x86
 include $(BUILD_PREBUILT)
-
-endif