lib/crypto: x86/blake2s: Reduce size of BLAKE2S_SIGMA2

lib/crypto: x86/blake2s: Reduce size of BLAKE2S_SIGMA2

Save 480 bytes of .rodata by replacing the .long constants with .bytes,
and using the vpmovzxbd instruction to expand them.

Also update the code to do the loads before incrementing %rax rather
than after.  This avoids the need for the first load to use an offset.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250827151131.27733-8-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>