perf tools: Use ACCESS_ONCE() instead of volatile cast

perf tools: Use ACCESS_ONCE() instead of volatile cast

Use ACCESS_ONCE() instead of the cast to volatile and read. This is just
a style change which is reader friendly.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1411484109-10442-1-git-send-email-bobby.prani@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>