[ARM] 5559/1: Limit the stack unwinding caused by a kthread exit

[ARM] 5559/1: Limit the stack unwinding caused by a kthread exit

When a kthread function returns, it branches to do_exit(). However, the
unwinding information isn't valid anymore and any stack trace caused by
do_exit() may be incorrect. This patch adds a kernel_thread_exit()
function and annotated with '.cantunwind' so that the unwinder stops
when reaching it.

Tested-by: Tony Lindgren <[email protected]>
Signed-off-by: Catalin Marinas <[email protected]>
Signed-off-by: Russell King <[email protected]>