Implement mblen(3).

Change-Id: I65948ea5b9ecd63f966ba767ad6db4a2effc4700
diff --git a/libc/libc.arm.map b/libc/libc.arm.map
index 420cd7e..4e69bf1 100644
--- a/libc/libc.arm.map
+++ b/libc/libc.arm.map
@@ -1279,6 +1279,7 @@
     getpwent;
     getsubopt;
     hasmntopt;
+    mblen;
     pthread_getname_np;
     quotactl;
     setdomainname;