ossl(4): Install the man page also for arm64

In 22bd0c9731d7 ossl(4) was ported to arm64. The manual page was
adapted, but never installed since the ossl(4) manual page was
i386 / amd64 only.

Reviewed by: mhorne
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D29762