Add simple debug hit rate counter

Add a very simple debug framework to
measure the hit rate of a given condition.

Simply insert macro

dbg_hit_on(x);

Anywhere you want to compute hit rate of condition x
and then call, as example in poll(), function
dbg_print_hit_rate() to print current results.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>