perf bench: Add format constants to bench.h for unified output formatting

This patch adds some constants and extern declaration to
bench.h. These are used for unified output formatting
of 'perf bench'.

Signed-off-by: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1257808802-9420-2-git-send-email-mitake@dcl.info.waseda.ac.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>