lang/zig: build for lowest common denominator CPU

Systems that don't have AVX2 or newer CPU instructions will SIGILL. This
seems particularly cruel for a language with the moniker "Gotta Go Fast".

PR: 269206
Differential Revision: https://reviews.freebsd.org/D38284

Reported by: Yonas Yanfa
Tested by: Oleh Vinichenko