libav IDCT speed tests

[Michael Hope]

Måns pointed me at the IDCT throughput test that's included with
libav.  I've written up a page on how to build and run it at:
 https://wiki.linaro.org/MichaelHope/Sandbox/LibAvDCT

Included are results with and without the vectoriser.  In all cases
the vectoriser improves things, including increasing the SIMPLE-C
version by 11 % and the peak by 17 %.

The coefficient of variance is low so the results are consistent.  I
haven't investigated the benchmark itself to see if its valid - we
could be vectorising the loop overhead instead of the IDCT itself.

-- Michael