[backport#12573] Fix compilation when compiler do not support __builtin_clz*

Summary:
18307849b Consensus: Fix bug when compiler do not support __builtin_clz* (532479301)

Pull request description:

#ifdef is not correct since defination is defined to 0 or 1. Should change to #if

Backport of Core PR12573

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, PiRK

Reviewed By: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8208