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-mips64/bionic/vfork.S b/libc/arch-mips64/bionic/vfork.S
index 5d457a9..b5ff5d4 100644
--- a/libc/arch-mips64/bionic/vfork.S
+++ b/libc/arch-mips64/bionic/vfork.S
@@ -42,6 +42,7 @@
 #endif
 
 LEAF(vfork,FRAMESZ)
+__BIONIC_WEAK_ASM_FOR_NATIVE_BRIDGE(vfork)
 #if FRAMESZ!=0
 	PTR_SUBU $sp, FRAMESZ
 #endif