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/clone.cpp b/libc/bionic/clone.cpp
index 66ec503..b895305 100644
--- a/libc/bionic/clone.cpp
+++ b/libc/bionic/clone.cpp
@@ -34,6 +34,7 @@
 
 #include "pthread_internal.h"
 
+#include "private/bionic_defs.h"
 #include "private/bionic_macros.h"
 
 extern "C" pid_t __bionic_clone(uint32_t flags, void* child_stack, int* parent_tid, void* tls, int* child_tid, int (*fn)(void*), void* arg);
@@ -52,10 +53,11 @@
   __exit(status);
 }
 
+__BIONIC_WEAK_FOR_NATIVE_BRIDGE
 int clone(int (*fn)(void*), void* child_stack, int flags, void* arg, ...) {
-  int* parent_tid = NULL;
-  void* new_tls = NULL;
-  int* child_tid = NULL;
+  int* parent_tid = nullptr;
+  void* new_tls = nullptr;
+  int* child_tid = nullptr;
 
   if (fn != nullptr && child_stack == nullptr) {
     errno = EINVAL;