libmd: fix assembly optimized skein implementation

The assembly implementation incorrectly used logical AND instead of
bitwise AND. Fix, and re-enable in libmd.

Submitted by: Yang Zhong <yzhong@freebsdfoundation.org>
Reviewed by: cem (earlier)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26614