Merge hardware POPCNT detection and use

Tests on Joona luxury iCore7 QUAD show that speed increase
against standrd 64bit routine is between 3% and 4%.

So it seems a good thing to have. Also the user feedback at
startup regarding the compile and the hardware detection can
be an useful debug tool.

No functional change.

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

diff --cc src/movegen.cpp
Simple merge