Add an ARM scalar optimized implementation of BLAKE2s.

NEON isn't very useful for BLAKE2s because the BLAKE2s block size is too
small for NEON to help.  Each NEON instruction would depend on the
previous one, resulting in poor performance.

With scalar instructions, on the other hand, we can take advantage of
ARM's "free" rotations (like I did in chacha-scalar-core.S) to get an
implementation get runs much faster than the C implementation.

Performance results on Cortex-A7 in cycles per byte using the shash API:

	4096-byte messages:
		blake2s-256-arm:     18.8
		blake2s-256-generic: 26.0

	500-byte messages:
		blake2s-256-arm:     20.3
		blake2s-256-generic: 27.9

	100-byte messages:
		blake2s-256-arm:     29.7
		blake2s-256-generic: 39.2

	32-byte messages:
		blake2s-256-arm:     50.6
		blake2s-256-generic: 66.2

Except on very short messages, this is still slower than the NEON
implementation of BLAKE2b which I've written; that is 14.0, 16.4, 25.8,
and 76.1 cpb on 4096, 500, 100, and 32-byte messages, respectively.
However, optimized BLAKE2s is useful for cases where BLAKE2s is used
instead of BLAKE2b, such as WireGuard.

This new implementation is added in the form of a new module
blake2s-arm.ko, which is analogous to blake2s-x86_64.ko in that it
provides blake2s_compress_arch() for use by the library API as well as
optionally register the algorithms with the shash API.
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Tested-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>