Implement mblen(3).

Change-Id: I65948ea5b9ecd63f966ba767ad6db4a2effc4700
diff --git a/libc/libc.mips64.map b/libc/libc.mips64.map
index bdde35e..7ea74a6 100644
--- a/libc/libc.mips64.map
+++ b/libc/libc.mips64.map
@@ -1201,6 +1201,7 @@
     getpwent;
     getsubopt;
     hasmntopt;
+    mblen;
     pthread_getname_np;
     quotactl;
     setdomainname;