Mark __BIONIC_WEAK_FOR_NATIVE_BRIDGE symbols

To make it easier for Native Bridge implementations
to override these symbols.

Bug: http://b/67993967
Test: make
Change-Id: I4c53e53af494bca365dd2b3305ab0ccc2b23ba44
diff --git a/libc/private/bionic_defs.h b/libc/private/bionic_defs.h
index d9e016c..1d4f86b 100644
--- a/libc/private/bionic_defs.h
+++ b/libc/private/bionic_defs.h
@@ -34,5 +34,6 @@
  * by native bridge implementation.
  */
 #define __BIONIC_WEAK_FOR_NATIVE_BRIDGE __attribute__((__weak__, __noinline__))
+#define __BIONIC_WEAK_VARIABLE_FOR_NATIVE_BRIDGE __attribute__((__weak__))
 
 #endif /* __BIONIC_PRIVATE_BIONIC_DEFS_H_ */