Enable accelerated AES-XTS software crypto in GENERIC.

In particular, using GELI on a root filesystem will only use
accelerated software crypto drivers if they are available before the
root filesystem is mounted. While these modules can be loaded from
the loader, including them in GENERIC provides a better out-of-the-box
experience for users.

Both aesni(4) and armv8crypto(4) provide accelerated implementations
of the default cipher used by GELI (AES-XTS) in addition to other
ciphers.

Reviewed by: mhorne, allanjude, markj
Differential Revision: https://reviews.freebsd.org/D28100