Patrice:
From that I thought Tor used already OpenSSL but it wasn't installed. :S
You had OpenSSL library installed as a shared object libcrypto.so to which tor is dynamically linked. Though you didn't have /usr/bin/openssl aka "OpenSSL command line tool". This is pretty common setup.
I bought this board with this CPU (incl. AES-NI support) because I thought it would give a benefit.
It's better to stick with more common techniques for ciphers, not with AES-specific. I mean vectorized operations in modern CPUs like AVX, AVX2, AVX512, NEON and even SSE3. Tor is gradually migrating to ChaCha20 instead of AES as stream cipher. ChaCha20 runs on vectorized operations in time comparable to AES with AES-NI and faster than AES w/o AES-NI since AES doesn't support vectorized operations. Also it's better to use different platforms in light of recent discussion about Intel ME and just because Tor needs diversity on all levels. :)
-- Ivan Markin