cardiff: Better benchmark setup reporting

The benchmark setup was badly printed. Let's move it to the "context"
block and add a title at the beginning.
cardiff: Renaming "global perf" to "mean perf"

In fact the deviance is compared againt the mean perf. The word
"global" wasn't precise enough
cardiff: Better cpu reporting

Let's report the <amount of cpu> x <model>.

The "x" makes the amount much more understandable and make the read
easier unless we have the feeling that 2 is part of the cpu model.