objtool: Add do_task_dead() to global noreturn list

objtool reports the following new warning:

  kernel/exit.o: warning: objtool: do_exit() falls through to next function complete_and_exit()

The warning is caused by do_exit()'s new call to do_task_dead(), which
is a new "noreturn" function which objtool doesn't know about yet,
introduced by:

  9af6528ee9b6 ("sched/core: Optimize __schedule()")

( objtool has to know all the global noreturn functions so it can follow
  the control flow of any functions which call them.  Unfortunately they
  need to be hard-coded because there's no automated way to detect them. )

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kbuild-all@01.org
Cc: tipbuild@zytor.com
Link: http://lkml.kernel.org/r/20160922212125.zbuewckqll4yur25@treble
Signed-off-by: Ingo Molnar <mingo@kernel.org>

tools/objtool/builtin-check.c

@@ -175,6 +175,7 @@ static int __dead_end_function(struct objtool_file *file, struct symbol *func,
 		"__stack_chk_fail",
 		"panic",
 		"do_exit",
+		"do_task_dead",
 		"__module_put_and_exit",
 		"complete_and_exit",
 		"kvm_spurious_fault",