kernel: crypto optimization

Linux doesn't have optimized crypto implementations for RISC-V. Storage encryption and dm/fs-verity are going to be unusably slow without optimized versions of AES and SHA-2 at least, preferably using [Scalar Cryptography extensions](https://github.com/riscv/riscv-crypto/releases/tag/v1.0.1-scalar) or the upcoming Vector Cryptography extension. There's some potential overlap here with issue #36.