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/arch-arm64/bionic/vfork.S b/libc/arch-arm64/bionic/vfork.S
index 92fa333..0a83cc7 100644
--- a/libc/arch-arm64/bionic/vfork.S
+++ b/libc/arch-arm64/bionic/vfork.S
@@ -31,6 +31,7 @@
 #include <linux/sched.h>
 
 ENTRY(vfork)
+__BIONIC_WEAK_ASM_FOR_NATIVE_BRIDGE(vfork)
     // __get_tls()[TLS_SLOT_THREAD_ID]->cached_pid_ = 0
     mrs     x0, tpidr_el0
     ldr     x0, [x0, #8]