The tkill(2) function was removed from libc in NDK r11.

diff -u -r ../bdwgc-c861ec3d1825b5bb450d20bf9091562fa8a81a4d/pthread_stop_world.c ./pthread_stop_world.c
--- ../bdwgc-c861ec3d1825b5bb450d20bf9091562fa8a81a4d/pthread_stop_world.c	2016-01-13 03:34:56.000000000 -0500
+++ ./pthread_stop_world.c	2016-03-10 07:09:03.607785056 -0500
@@ -450,7 +450,7 @@
 #endif
 
 #ifdef USE_TKILL_ON_ANDROID
-  extern int tkill(pid_t tid, int sig); /* from sys/linux-unistd.h */
+  static int tkill(pid_t tid, int sig) { return tgkill(-1, tid, sig); }
 
   static int android_thread_kill(pid_t tid, int sig)
   {