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/bionic/fchmodat.cpp b/libc/bionic/fchmodat.cpp
index c28e15a..1f83c4b 100644
--- a/libc/bionic/fchmodat.cpp
+++ b/libc/bionic/fchmodat.cpp
@@ -34,7 +34,7 @@
 
 #include "private/ErrnoRestorer.h"
 
-extern "C" int __fchmodat(int, const char*, mode_t);
+extern "C" int ___fchmodat(int, const char*, mode_t);
 
 int fchmodat(int dirfd, const char* pathname, mode_t mode, int flags) {
   if ((flags & ~AT_SYMLINK_NOFOLLOW) != 0) {
@@ -63,5 +63,5 @@
     return result;
   }
 
-  return __fchmodat(dirfd, pathname, mode);
+  return ___fchmodat(dirfd, pathname, mode);
 }