Hide accidentally-exposed __clock_nanosleep.

Bug: http://b/21858067
Change-Id: Iaa83a5e17cfff796aed4f641d0d14427614d9399
diff --git a/libc/arch-arm/syscalls/__clock_nanosleep.S b/libc/arch-arm/syscalls/___clock_nanosleep.S
similarity index 78%
rename from libc/arch-arm/syscalls/__clock_nanosleep.S
rename to libc/arch-arm/syscalls/___clock_nanosleep.S
index ba7ffc4..31420bb 100644
--- a/libc/arch-arm/syscalls/__clock_nanosleep.S
+++ b/libc/arch-arm/syscalls/___clock_nanosleep.S
@@ -2,7 +2,7 @@
 
 #include <private/bionic_asm.h>
 
-ENTRY(__clock_nanosleep)
+ENTRY(___clock_nanosleep)
     mov     ip, r7
     ldr     r7, =__NR_clock_nanosleep
     swi     #0
@@ -11,4 +11,5 @@
     bxls    lr
     neg     r0, r0
     b       __set_errno_internal
-END(__clock_nanosleep)
+END(___clock_nanosleep)
+.hidden ___clock_nanosleep