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/stdlib/exit.c b/libc/stdlib/exit.c
index 510cb83..e301a2a 100644
--- a/libc/stdlib/exit.c
+++ b/libc/stdlib/exit.c
@@ -29,9 +29,12 @@
 
 #include <unistd.h>
 
+#include "private/bionic_defs.h"
+
 extern void __cxa_finalize(void* dso_handle);
 extern void __cxa_thread_finalize();
 
+__BIONIC_WEAK_FOR_NATIVE_BRIDGE
 void exit(int status) {
   __cxa_thread_finalize();
   __cxa_finalize(NULL);