arm64: ftrace: add save_stack_trace_regs()

Currently, enabling stacktrace of a kprobe events generates warning:

  echo stacktrace > /sys/kernel/debug/tracing/trace_options
  echo "p xhci_irq" > /sys/kernel/debug/tracing/kprobe_events
  echo 1 > /sys/kernel/debug/tracing/events/kprobes/enable

save_stack_trace_regs() not implemented yet.
------------[ cut here ]------------
WARNING: CPU: 1 PID: 0 at ../kernel/stacktrace.c:74 save_stack_trace_regs+0x3c/0x48
Modules linked in:

CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.8.0-rc4-dirty #5128
Hardware name: ARM Juno development board (r1) (DT)
task: ffff800975dd1900 task.stack: ffff800975ddc000
PC is at save_stack_trace_regs+0x3c/0x48
LR is at save_stack_trace_regs+0x3c/0x48
pc : [<ffff000008126c64>] lr : [<ffff000008126c64>] pstate: 600003c5
sp : ffff80097ef52c00

Call trace:
   save_stack_trace_regs+0x3c/0x48
   __ftrace_trace_stack+0x168/0x208
   trace_buffer_unlock_commit_regs+0x5c/0x7c
   kprobe_trace_func+0x308/0x3d8
   kprobe_dispatcher+0x58/0x60
   kprobe_breakpoint_handler+0xbc/0x18c
   brk_handler+0x50/0x90
   do_debug_exception+0x50/0xbc

This patch implements save_stack_trace_regs(), so that stacktrace of a
kprobe events can be obtained.

After this patch, there is no warning and we can see the stacktrace for
kprobe events in trace buffer.

more /sys/kernel/debug/tracing/trace
          <idle>-0     [004] d.h.  1356.000496: p_xhci_irq_0:(xhci_irq+0x0/0x9ac)
          <idle>-0     [004] d.h.  1356.000497: <stack trace>
  => xhci_irq
  => __handle_irq_event_percpu
  => handle_irq_event_percpu
  => handle_irq_event
  => handle_fasteoi_irq
  => generic_handle_irq
  => __handle_domain_irq
  => gic_handle_irq
  => el1_irq
  => arch_cpu_idle
  => default_idle_call
  => cpu_startup_entry
  => secondary_start_kernel
  =>

Tested-by: David A. Long <dave.long@linaro.org>
Reviewed-by: James Morse <james.morse@arm.com>
Signed-off-by: Pratyush Anand <panand@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>