devel/cpu_features: port to powerpc*

This adds the relevant bits of polyfill, permitting the
library to build on powerpc* (only powerpc64le tested).

Approved by: portmgr (build fix blanket)
MFH: 2026Q1
Sponsored by: Raptor Computing Systems, LLC

(cherry picked from commit 05dffccdf23880277fdcec143ccef78f254e10ea)