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/fork.cpp b/libc/bionic/fork.cpp
index efcbb8c..33b7343 100644
--- a/libc/bionic/fork.cpp
+++ b/libc/bionic/fork.cpp
@@ -28,8 +28,10 @@
 
 #include <unistd.h>
 
+#include "private/bionic_defs.h"
 #include "pthread_internal.h"
 
+__BIONIC_WEAK_FOR_NATIVE_BRIDGE
 int fork() {
   __bionic_atfork_run_prepare();