The Blake2b hash class (BLAKE2b-512) was fully implemented but never
registered in Hash::GetAvailableAlgorithms(), so it was absent from the
Random Pool Enrichment, Keyfile Generator and Benchmark dialogs on
Linux/macOS. In particular, when Argon2 is selected as the volume PRF,
the RNG pool hash is set to BLAKE2b-512, but the Random Pool Enrichment
dialog could neither display nor pre-select it, diverging from Windows
which offers BLAKE2b-512 in the same dialog.
Register Blake2b after Streebog (matching the Crypto.c PRF order). Its
64-byte digest divides RNG_POOL_SIZE (320), so the HashMixPool size
constraint holds, and blake2b.o is always built in non-wolfCrypt builds.
Also make Pkcs5Kdf::GetAlgorithm(const Hash&) Argon2-aware by removing
the unconditional skip of the Argon2 KDF, so a BLAKE2b-512 hash now maps
to the Argon2 KDF instead of throwing.