Remove unnecessary instructions from x86/x86_64 syscalls.

__set_errno returns -1 exactly so that callers don't need to bother.
The other architectures were already taking advantage of this, but
no one had ever fixed x86 and x86_64.

Change-Id: Ie131494be664f6c4a1bbf8c61bbbed58eac56122
diff --git a/libc/arch-x86/bionic/vfork.S b/libc/arch-x86/bionic/vfork.S
index ec6f6ca..ffa6b16 100644
--- a/libc/arch-x86/bionic/vfork.S
+++ b/libc/arch-x86/bionic/vfork.S
@@ -39,7 +39,6 @@
   negl    %eax
   pushl   %eax
   call    __set_errno
-  orl     $-1, %eax
 1:
-    jmp     *%ecx  // Jump to the stored return address.
+  jmp     *%ecx  // Jump to the stored return address.
 END(vfork)