Fix assembler warnings.

clang complains if you define a symbol and _then_ make it weak, rather
than the other way round:

  /tmp/setjmp-c3c977.s:90:1: warning: sigsetjmp changed binding to STB_WEAK
  .weak sigsetjmp;
  ^

Test: treehugger
Change-Id: Iee6b0ea456bb2e92aea810ce45f171caabaa89d2
diff --git a/libc/arch-arm64/bionic/vfork.S b/libc/arch-arm64/bionic/vfork.S
index 9b19232..addb775 100644
--- a/libc/arch-arm64/bionic/vfork.S
+++ b/libc/arch-arm64/bionic/vfork.S
@@ -32,8 +32,7 @@
 #include <asm/signal.h>
 #include <linux/sched.h>
 
-ENTRY(vfork)
-__BIONIC_WEAK_ASM_FOR_NATIVE_BRIDGE(vfork)
+ENTRY_WEAK_FOR_NATIVE_BRIDGE(vfork)
     // x9 = __get_tls()[TLS_SLOT_THREAD_ID]
     mrs     x9, tpidr_el0
     ldr     x9, [x9, #(TLS_SLOT_THREAD_ID * 8)]