Adding --ldlat option to specify desired latency for loads event.

Specify 50 as loads event latency:

  $ perf mem record -e ldlat-loads -v --ldlat 50 true
  calling: record -W -d -e cpu/mem-loads,ldlat=50/P true
Signed-off-by: NJiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1465928361-2442-2-git-send-email-jolsa@kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>