Return "none" if no matching arch found

So result of get-prebuilt-src-arch can be passed to
LOCAL_MODULE_TARGET_ARCH, which takes empty string as "any" actually.

Change-Id: I916c9738ccce4a94ac084fb4141d54659e896a1f
diff --git a/core/definitions.mk b/core/definitions.mk
index 3bded9f..d7d8b25 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -2187,11 +2187,13 @@
 $(if $(call if-build-from-source,$(2),$(3)),$(eval include $(1)))
 endef
 
-## Return the arch for the source file of a prebuilt
+# Return the arch for the source file of a prebuilt
+# Return "none" if no matching arch found, so the result can be passed to
+# LOCAL_MODULE_TARGET_ARCH.
 # $(1) the list of archs supported by the prebuilt
 define get-prebuilt-src-arch
 $(strip $(if $(filter $(TARGET_ARCH),$(1)),$(TARGET_ARCH),\
-  $(if $(filter $(TARGET_2ND_ARCH),$(1)),$(TARGET_2ND_ARCH))))
+  $(if $(filter $(TARGET_2ND_ARCH),$(1)),$(TARGET_2ND_ARCH),none)))
 endef
 
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