Allow tkill

tkill and tgkill are syscalls that do not have glibc wrappers, but
they are required in order to send a signal to a specific thread.
Non-android software may call them directly with syscall().  Bionic
provides a wrapper for tgkill, so seccomp allows it, but not for
tkill.  Add tkill to the whitelist.

This can be reproduced with:
sleep 1000 & sleep 1 ; strace -p $!
then hit ctrl-C, and the shell will print "Bad system call" because
the strace process died with SIGSYS

Bug: 34586922
Test: repro case is fixed
Change-Id: Ib6962a967f2cc757f2906de7905e75e1b6d6f39f