the timing facilities in LKET

[Gui,Jian]

Based on the discussions a couple of days ago, we plan to implement
the timing facilities as follow:

1) keep current do_gettimeofday;
2) Use get_cycles, trace the variation of cpu frequency and let the
postprocessing translate cycles into seconds.microseconds.

The default is to use get_cycles, but we can also change it like:

probe begin
{
   set_timing_method(2); //e.g. get_cycles is 1, do_gettimeofday is 2
}

Actually, sched_clock should be a better choice but it is not exported
out of kernel yet. We'd like to use get_cycles first and see how to
integrate sched_clock into LKET/systemtap better.

Any comments? Thanks.