perf stat: Add --pre and --post command

In order to measure kernel builds, one has to do some pre/post cleanup
work in order to do the repeat build.

So provide --pre and --post command hooks to allow doing just that.

  perf stat --repeat 10 --null --sync --pre 'make -s O=defconfig-build/clean' \
	-- make -s -j64 O=defconfig-build/ bzImage

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Stephane Eranian <eranian@gmail.com>
Link: http://lkml.kernel.org/r/1350992414.13456.5.camel@twins
[ committer note: Added respective entries in Documentation/perf-stat.txt ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>