Add missing cfi directives for x86 assembler.

Change-Id: I80d3e33a71bbaeab5f39b667ebe61e865fd54b80
diff --git a/libc/arch-x86/bionic/vfork.S b/libc/arch-x86/bionic/vfork.S
index 6c02910..ca7af0f 100644
--- a/libc/arch-x86/bionic/vfork.S
+++ b/libc/arch-x86/bionic/vfork.S
@@ -32,6 +32,8 @@
 
 ENTRY(vfork)
   popl    %ecx  // Grab the return address.
+  .cfi_adjust_cfa_offset 4
+  .cfi_rel_offset ecx, 0
   movl    $__NR_vfork, %eax
   int     $0x80
   cmpl    $-MAX_ERRNO, %eax