This is a new regression from x265 master that assumes all Unix is using ELF binary which causes this build error on macOS.

I reported this already: https://bitbucket.org/multicoreware/x265_git/issues/980/source-common-x86-cpu-aasm-assumes-all

You can revert commit f5e4a648727ea86828df236eb44742fe3e3bf366 to workaround this for now if you want to build master.