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-arm/syscalls/___fchmod.S

@@ -2,7 +2,7 @@
 
 #include <private/bionic_asm.h>
 
-ENTRY(__fchmod)
+ENTRY(___fchmod)
     mov     ip, r7
     ldr     r7, =__NR_fchmod
     swi     #0
@@ -11,4 +11,5 @@ ENTRY(__fchmod)
     bxls    lr
     neg     r0, r0
     b       __set_errno_internal
-END(__fchmod)
+END(___fchmod)
+.hidden ___fchmod

libc/arch-arm/syscalls/___fchmodat.S

@@ -2,7 +2,7 @@
 
 #include <private/bionic_asm.h>
 
-ENTRY(__fchmodat)
+ENTRY(___fchmodat)
     mov     ip, r7
     ldr     r7, =__NR_fchmodat
     swi     #0
@@ -11,4 +11,5 @@ ENTRY(__fchmodat)
     bxls    lr
     neg     r0, r0
     b       __set_errno_internal
-END(__fchmodat)
+END(___fchmodat)
+.hidden ___fchmodat