modifty sleepers to have easier evaluation of interaction between cpu freq scaling

modifty sleepers to have easier evaluation of interaction between cpu freq scaling
and scheduler lock (pipe based or futex based)

See http://www.valgrind.org/docs/manual/manual-core.html#manual-core.pthreads_perf_sched
for background info about cpu freq scaling and valgrind thread scheduler.

To reproduce the interaction, do:
for sched in --fair-sched=yes --fair-sched=no
do
  for affinity in 0 1
  do
    echo $sched $affinity
    time ./vg-in-place $sched -q ./gdbserver_tests/sleepers 1000000 0 1000 B-B-B-B- $affinity
  done
done

which gives the below output (intel core i5-6402P, debian 8, kernel  3.16.0).
In summary: the fair scheduler is fair, the pipe based scheduler
can be really unfair (e.g. with --fair-sched=no and no affinity,
2 threads are finishing their work, while the 2 other threads are
starting their work only after the first 2 have fully finished).
The difference in timing is significant : 1m14s versus around 47 seconds.

  Note: If the governor is set to performance, strangely, the time needed for
  --fair-sched=no increases slighltly (to around 48 seconds).
  The time for --fair-sched=yes with or without affinity is then also
  to around 48 seconds.

Below is timing with on-demand governor:

--fair-sched=yes 0
loops/sleep_ms/burn/threads_spec/affinity:  1000000 0 1000 B-B-B-B- 0
Brussels ready to sleep and/or burn
London ready to sleep and/or burn
Petaouchnok ready to sleep and/or burn
main ready to sleep and/or burn
Brussels finished to sleep and/or burn
London finished to sleep and/or burn
Petaouchnok finished to sleep and/or burn
main finished to sleep and/or burn

real	1m14.582s
user	1m14.348s
sys	0m0.204s
--fair-sched=yes 1
loops/sleep_ms/burn/threads_spec/affinity:  1000000 0 1000 B-B-B-B- 1
Brussels ready to sleep and/or burn
London ready to sleep and/or burn
Petaouchnok ready to sleep and/or burn
main ready to sleep and/or burn
main finished to sleep and/or burn
Brussels finished to sleep and/or burn
Petaouchnok finished to sleep and/or burn
London finished to sleep and/or burn

real	0m46.785s
user	0m46.756s
sys	0m0.032s
--fair-sched=no 0
loops/sleep_ms/burn/threads_spec/affinity:  1000000 0 1000 B-B-B-B- 0
Brussels ready to sleep and/or burn
Brussels finished to sleep and/or burn
London ready to sleep and/or burn
London finished to sleep and/or burn
Petaouchnok ready to sleep and/or burn
main ready to sleep and/or burn
Petaouchnok finished to sleep and/or burn
main finished to sleep and/or burn

real	0m47.742s
user	0m48.224s
sys	0m0.084s
--fair-sched=no 1
loops/sleep_ms/burn/threads_spec/affinity:  1000000 0 1000 B-B-B-B- 1
Brussels ready to sleep and/or burn
London ready to sleep and/or burn
Petaouchnok ready to sleep and/or burn
main ready to sleep and/or burn
Brussels finished to sleep and/or burn
London finished to sleep and/or burn
main finished to sleep and/or burn
Petaouchnok finished to sleep and/or burn

real	0m46.601s
user	0m46.568s
sys	0m0.036s

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16251