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

libc/arch-mips/syscalls/___fchmodat.S

@@ -0,0 +1,20 @@
+/* Generated by gensyscalls.py. Do not edit. */
+
+#include <private/bionic_asm.h>
+
+ENTRY(___fchmodat)
+    .set noreorder
+    .cpload t9
+    li v0, __NR_fchmodat
+    syscall
+    bnez a3, 1f
+    move a0, v0
+    j ra
+    nop
+1:
+    la t9,__set_errno_internal
+    j t9
+    nop
+    .set reorder
+END(___fchmodat)
+.hidden ___fchmodat