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/tools/gensyscalls.py

@@ -286,8 +286,9 @@ def add_footer(pointer_length, stub, syscall):
     for alias in aliases:
         stub += function_alias % { "func" : syscall["func"], "alias" : alias }
 
-    # Use hidden visibility for any functions beginning with underscores.
-    if pointer_length == 64 and syscall["func"].startswith("__"):
+    # Use hidden visibility on LP64 for any functions beginning with underscores.
+    # Force hidden visibility for any functions which begin with 3 underscores
+    if (pointer_length == 64 and syscall["func"].startswith("__")) or syscall["func"].startswith("___"):
         stub += '.hidden ' + syscall["func"] + '\n'
 
     return stub