Add accelerated arm64 sha512 to libmd

As with sha256 add support for accelerated sha512 support to libmd on
arm64. This depends on clang 13+ to build as this is the first release
with the needed intrinsics. Gcc should also support them, however from
a currently unknown release.

Reviewed by: cem
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33373