Hide accidentally-exposed __clock_nanosleep.

Bug: http://b/21858067
Change-Id: Iaa83a5e17cfff796aed4f641d0d14427614d9399
diff --git a/libc/arch-arm64/syscalls/___clock_nanosleep.S b/libc/arch-arm64/syscalls/___clock_nanosleep.S
new file mode 100644
index 0000000..0dcfd4f
--- /dev/null
+++ b/libc/arch-arm64/syscalls/___clock_nanosleep.S
@@ -0,0 +1,15 @@
+/* Generated by gensyscalls.py. Do not edit. */
+
+#include <private/bionic_asm.h>
+
+ENTRY(___clock_nanosleep)
+    mov     x8, __NR_clock_nanosleep
+    svc     #0
+
+    cmn     x0, #(MAX_ERRNO + 1)
+    cneg    x0, x0, hi
+    b.hi    __set_errno_internal
+
+    ret
+END(___clock_nanosleep)
+.hidden ___clock_nanosleep