[avalanche] Use black magic for popcount.

Summary:
Or compiler intrinsic if these are supported.

The black magic algorithm is from 'Owen Anderson <resistor@mac.com>', as far as I know.

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox, Fabien, Mengerian

Reviewed By: #bitcoin_abc, jasonbcox, Fabien, Mengerian

Subscribers: Mengerian, jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2148