remove glibc < 2.27 compat code

Summary:
2.27 introduced a new version of log2f and is now the oldest supported version of glibc after D13460

_divmoddi4 is a feature from GCC7 and we already enforce 8.3.

This is a partial backport of core#22930

Test Plan:
ninja all check-all
gitian builds

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13472