lib/crypto: riscv/aes-xts: Migrate optimized code into library

Instead of exposing the riscv-optimized AES-XTS code via a
riscv-specific crypto_skcipher algorithm, just implement the AES-XTS
library functions.  This is simpler, it makes the AES-XTS library
functions be riscv-optimized, and it also fixes the longstanding issue
where the riscv-optimized AES-XTS code was disabled by default.  AES-XTS
support still remains available through crypto_skcipher via
crypto/aes.c, but individual architectures no longer need to handle it.

To match what the library expects, update the assembly functions to
operate on struct aes_key rather than struct crypto_aes_ctx, adjust the
argument order, and remove the redundant ciphertext stealing support
which is already implemented in a generic way in the library.

Bump up the priority of the corresponding library-based algorithm on
riscv now that it no longer has to be lower than arch/riscv/crypto/.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>