Make malloc debug symbols platform-private

Bug: http://b/28432442
Change-Id: I96b9cb8b57726b4e8d53069e72666cf107cdeba2
diff --git a/libc/libc.arm64.map b/libc/libc.arm64.map
index b1a50ae..bdde35e 100644
--- a/libc/libc.arm64.map
+++ b/libc/libc.arm64.map
@@ -552,10 +552,6 @@
     madvise;
     mallinfo;
     malloc;
-    malloc_backtrace;
-    malloc_disable;
-    malloc_enable;
-    malloc_iterate;
     malloc_info;
     malloc_usable_size;
     mbrlen;
@@ -1234,4 +1230,8 @@
     android_net_res_stats_get_info_for_net;
     android_net_res_stats_aggregate;
     android_net_res_stats_get_usable_servers;
+    malloc_backtrace;
+    malloc_disable;
+    malloc_enable;
+    malloc_iterate;
 } LIBC_O;