perf tools: Add dump_stack function

To help in debugging the tools, provides functionality roughly similar
to the function with the same name in the kernel.

Copied from glibc backtrace function man page.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-6nw2sak21bqy8h1m2syyo816@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/util.h

@@ -266,4 +266,6 @@ size_t hex_width(u64 v);
 
 char *rtrim(char *s);
 
+void dump_stack(void);
+
 #endif