There is an updated pull request by q66 against master on the void-packages repository

https://github.com/void-ppc64/void-packages x265
https://github.com/void-linux/void-packages/pull/11540

x265: add options for altivec and assembly
This lets the user choose whether to build them in. Default to on for altivec on ppc64le (because that's always power8 or better) and off on ppc64 (because we support old hardware), assembly is on for ARM and x86 and off for everything else.

A patch file from https://github.com/void-linux/void-packages/pull/11540.patch is attached