ossl: Add support for armv7

[markj]

Summary

OpenSSL provides implementations of several AES modes which use
bitslicing and can be accelerated on CPUs which support the NEON
extension. This patch adds arm platform support to ossl and provides an
AES-CBC implementation, though bsaes_cbc_encrypt() only implements
decryption. The real goal is to provide an accelerated AES-GCM
implementation; this will be added in a subsequent patch.

Initially derived from https://reviews.freebsd.org/D37420.

Sponsored by: Klara, Inc.
Sponsored by: Stormshield