Fix BLAKE3 aarch64 assembly for FreeBSD and macOS

The x18 register isn't useable within FreeBSD kernel space, so we
have to fix the BLAKE3 aarch64 assembly for not using it.

The source files are here: https://github.com/mcmilk/BLAKE3-tests

Reviewed-by: Kyle Evans <kevans@FreeBSD.org>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #14728