Hide '__isthreaded' for lp64 libc.so

Bug: http://b/26805673
Change-Id: I2928d68287894be9f9ae7ed1858af512d8c80a3b
diff --git a/libc/libc.map.txt b/libc/libc.map.txt
index 3179049..71c16d9 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -62,7 +62,7 @@
     __isnormal;
     __isnormalf;
     __isnormall;
-    __isthreaded;
+    __isthreaded; # arm x86 mips
     __libc_current_sigrtmax;
     __libc_current_sigrtmin;
     __libc_init;