rs: support multilib builds

Generate source files into $(local-generated-sources-dir)
Restrict which architectures try to build using LOCAL_MODULE_TARGET_ARCH

Change-Id: Ibb1049c248ed4bcec32b81771f1f9fbcce66a390
diff --git a/cpu_ref/Android.mk b/cpu_ref/Android.mk
index fb55d30..f377486 100644
--- a/cpu_ref/Android.mk
+++ b/cpu_ref/Android.mk
@@ -13,6 +13,7 @@
 include $(CLEAR_VARS)
 LOCAL_CLANG := true
 LOCAL_MODULE := libRSCpuRef
+LOCAL_MODULE_TARGET_ARCH := arm mips x86 x86_64
 
 LOCAL_SRC_FILES:= \
 	rsCpuCore.cpp \