lib/crypto: x86/blake2s: Improve readability

lib/crypto: x86/blake2s: Improve readability

Various cleanups for readability.  No change to the generated code:

- Add some comments
- Add #defines for arguments
- Rename some labels
- Use decimal constants instead of hex where it makes sense.
  (The pshufd immediates intentionally remain as hex.)
- Add blank lines when there's a logical break

The round loop still could use some work, but this is at least a start.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20251102234209.62133-5-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>