perf probe: Show accessible local variables

Add -V (--vars) option for listing accessible local variables at given probe
point. This will help finding which local variables are available for event
arguments.

e.g.)
 # perf probe -V call_timer_fn:23
 Available variables at call_timer_fn:23
         @<run_timer_softirq+345>
                 function_type*  fn
                 int     preempt_count
                 long unsigned int       data
                 struct list_head        work_list
                 struct list_head*       head
                 struct timer_list*      timer
                 struct tvec_base*       base

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <20101021101323.3542.40282.stgit@ltc236.sdl.hitachi.co.jp>
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-probe.txt

@@ -17,6 +17,8 @@ or
 'perf probe' --list
 or
 'perf probe' --line='FUNC[:RLN[+NUM|:RLN2]]|SRC:ALN[+NUM|:ALN2]'
+or
+'perf probe' --vars='PROBEPOINT'
 
 DESCRIPTION
 -----------
@@ -57,6 +59,11 @@ OPTIONS
 	Show source code lines which can be probed. This needs an argument
 	which specifies a range of the source code. (see LINE SYNTAX for detail)
 
+-V::
+--vars=::
+	Show available local variables at given probe point. The argument
+	syntax is same as PROBE SYNTAX, but NO ARGs.
+
 -f::
 --force::
 	Forcibly add events with existing name.