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/bionic/environ.cpp b/libc/bionic/environ.cpp
index 363c1fd..243af62 100644
--- a/libc/bionic/environ.cpp
+++ b/libc/bionic/environ.cpp
@@ -28,6 +28,8 @@
 
 #include <unistd.h>
 
+#include "private/bionic_defs.h"
 // Keep that variable in separate .o file to make sure programs which define
 // their own "environ" are compileable.
+__BIONIC_WEAK_VARIABLE_FOR_NATIVE_BRIDGE
 char** environ;