Hide accidentally-exposed __clock_nanosleep.

Bug: http://b/21858067
Change-Id: Iaa83a5e17cfff796aed4f641d0d14427614d9399
diff --git a/libc/arch-mips64/syscalls/__clock_nanosleep.S b/libc/arch-mips64/syscalls/___clock_nanosleep.S
similarity index 82%
rename from libc/arch-mips64/syscalls/__clock_nanosleep.S
rename to libc/arch-mips64/syscalls/___clock_nanosleep.S
index 204675f..e9fb316 100644
--- a/libc/arch-mips64/syscalls/__clock_nanosleep.S
+++ b/libc/arch-mips64/syscalls/___clock_nanosleep.S
@@ -2,7 +2,7 @@
 
 #include <private/bionic_asm.h>
 
-ENTRY(__clock_nanosleep)
+ENTRY(___clock_nanosleep)
     .set push
     .set noreorder
     li v0, __NR_clock_nanosleep
@@ -22,5 +22,5 @@
     j t9
     move ra, t0
     .set pop
-END(__clock_nanosleep)
-.hidden __clock_nanosleep
+END(___clock_nanosleep)
+.hidden ___clock_nanosleep