ARM: 8049/1: ftrace/add save_stack_trace_regs() implementation

When configure kprobe events of ftrace with "stacktrace" option enabled
in arm, there is no stacktrace was recorded after the kprobe event was
triggered. The root cause is no save_stack_trace_regs() function implemented.

Implement the save_stack_trace_regs() function in arm, then ftrace will
call this architecture-related function to record the stacktrace into
ring buffer.

After this fix, stacktrace can be recorded, for example:

 # mount -t debugfs nodev /sys/kernel/debug
 # echo "p:netrx net_rx_action" >> /sys/kernel/debug/tracing/kprobe_events
 # echo 1 > /sys/kernel/debug/tracing/events/kprobes/netrx/enable
 # echo 1 > /sys/kernel/debug/tracing/options/stacktrace
 # echo 1 > /sys/kernel/debug/tracing/tracing_on
 # ping 127.0.0.1 -c 1
 # echo 0 > /sys/kernel/debug/tracing/tracing_on

 # cat /sys/kernel/debug/tracing/trace
 # tracer: nop
 #
 # entries-in-buffer/entries-written: 12/12   #P:1
 #
 #                              _-----=> irqs-off
 #                             / _----=> need-resched
 #                            | / _---=> hardirq/softirq
 #                            || / _--=> preempt-depth
 #                            ||| /     delay
 #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
 #              | |       |   ||||       |         |
             <------ missing some entries ---------------->
             ping-1200  [000] dNs1   667.603250: netrx: (net_rx_action+0x0/0x1f8)
             ping-1200  [000] dNs1   667.604738: <stack trace>
  => net_rx_action
  => do_softirq
  => local_bh_enable
  => ip_finish_output
  => ip_output
  => ip_local_out
  => ip_send_skb
  => ip_push_pending_frames
  => raw_sendmsg
  => inet_sendmsg
  => sock_sendmsg
  => SyS_sendto
  => ret_fast_syscall

Signed-off-by: Lin Yongting <linyongting@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>