Ensure raw fchmod/fchmodat syscalls are hidden.

In https://android-review.googlesource.com/#/c/127908/5/libc/SYSCALLS.TXT@116
Elliott said:

  for LP64 these will be hidden. for LP32 we were cowards and left
  them all public for compatibility (though i don't think we ever
  dremeled to see whether it was needed). we don't have an easy
  way to recognize additions, though, so we can't prevent adding
  new turds.

Add a mechanism to prevent the adding of new turds, and use that
mechanism on the fchmod/fchmodat system calls.

Bug: 19233951
Change-Id: I98f98345970b631a379f348df57858f9fc3d57c0
diff --git a/libc/arch-x86/syscalls/___fchmod.S b/libc/arch-x86/syscalls/___fchmod.S
new file mode 100644
index 0000000..119a695
--- /dev/null
+++ b/libc/arch-x86/syscalls/___fchmod.S
@@ -0,0 +1,27 @@
+/* Generated by gensyscalls.py. Do not edit. */
+
+#include <private/bionic_asm.h>
+
+ENTRY(___fchmod)
+    pushl   %ebx
+    .cfi_def_cfa_offset 8
+    .cfi_rel_offset ebx, 0
+    pushl   %ecx
+    .cfi_adjust_cfa_offset 4
+    .cfi_rel_offset ecx, 0
+    mov     12(%esp), %ebx
+    mov     16(%esp), %ecx
+    movl    $__NR_fchmod, %eax
+    int     $0x80
+    cmpl    $-MAX_ERRNO, %eax
+    jb      1f
+    negl    %eax
+    pushl   %eax
+    call    __set_errno_internal
+    addl    $4, %esp
+1:
+    popl    %ecx
+    popl    %ebx
+    ret
+END(___fchmod)
+.hidden ___fchmod