internal/cpu: enable arm64 SHA512 detection for freebsd/openbsd

internal/cpu: enable arm64 SHA512 detection for freebsd/openbsd

Change-Id: I1f21654b50d7b0cd8e1f854efe2724b72f067449
Reviewed-on: https://go-review.googlesource.com/c/go/+/422216
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Yuval Pavel Zholkover <paulzhol@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>