ossl: Add AES-GCM support for NEON-enabled armv7

This provides substantially higher throughput than the fallback
implementation.

Reviewed by: jhb
MFC after: 3 months
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D41305

(cherry picked from commit 629a72376d51aad812d6f1279403bc81c38c35d2)