Add an arm64 sha512 feature check

This is only available in recent toolchains. Check for it so we can
use it in libmd.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33166

(cherry picked from commit 629b5e87ed82aa428be1abbe51b80fa338fa839a)