Choosing the timer to use in benchmarking

[Michael Hope]

Being able to accurately and consistently measure the elapsed CPU time
is important for toolchain benchmarking. I ran a few experiments today
and wrote up the results at:
 https://wiki.linaro.org/WorkingGroups/ToolChain/Benchmarks/TimerAccuracy

The original is available at:
 http://bazaar.launchpad.net/~michaelh1/linaro-toolchain-benchmarks/trunk/view/head:/reports/timer-accuracy.rst

Short story:
 * Use clock_gettime(CLOCK_PROCESS_CPUTIME_ID, ...)
 * The mean is unaffected by CPU load
 * I/O load has a significant effect
 * Use nice -n -20 to reduce the variance

For a CPU bound, non-VFP, L1 bound, 5 s long program:
 * The variation coefficient is < 0.01 % so we can reliably measure
0.1 % changes
 * The accuracy is around 50 us

I've changed eembc-linaro and will change denbench-linaro next.  I
recommend anyone else measuring time on core based benchmarks to do
the same.

-- Michael