refactor architecture-specific popcount code

When Who What
2026-01-21 20:23:59 Nathan Bossart (bossartn) Closed in commitfest PG19-4 with status: Committed
2026-01-21 20:23:59 Nathan Bossart (bossartn) Moved from CF PG19-Final to CF PG19-4
2026-01-16 21:37:03 Nathan Bossart (bossartn) Added bossartn as committer
2026-01-16 21:37:02 Nathan Bossart (bossartn) New status: Ready for Committer
2026-01-14 20:40:21 Nathan Bossart (bossartn) Attached mail thread aWf_InS1VrbeXAfP@nathan
2026-01-14 20:40:20 Nathan Bossart (bossartn) Created patch record