lib/crypto: blake2s: Move generic code into blake2s.c

lib/crypto: blake2s: Move generic code into blake2s.c

Move blake2s_compress_generic() from blake2s-generic.c to blake2s.c.

For now it's still guarded by CONFIG_CRYPTO_LIB_BLAKE2S_GENERIC, but
this prepares for changing it to a 'static __maybe_unused' function and
just using the compiler to automatically decide its inclusion.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250827151131.27733-11-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>