Use double rotate for magic generation

Use double rotate for magic generation

Allow to choose among 4096 instances of pseudo-random
sequences instead of the previous 64 so the probability
to find a better sequence increases and actually we have
a much better 64 bit case and we can also use the 64 bit
version of pick_magic() also for 32 bits and althoug sub
optimal, because now we can have more choices results are
even slightly better also for 32 bit.

Use also a faster submask().

No functional change.

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