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-x86/bionic/vfork.S b/libc/arch-x86/bionic/vfork.S
index 231a36e..872d0c2 100644
--- a/libc/arch-x86/bionic/vfork.S
+++ b/libc/arch-x86/bionic/vfork.S
@@ -31,8 +31,7 @@
 
 // This custom code preserves the return address across the system call.
 
-ENTRY(vfork)
-__BIONIC_WEAK_ASM_FOR_NATIVE_BRIDGE(vfork)
+ENTRY_WEAK_FOR_NATIVE_BRIDGE(vfork)
   popl    %ecx  // Grab the return address.
   .cfi_adjust_cfa_offset 4
   .cfi_rel_offset ecx, 0