RISC-V: Implement (Zkne or Zknd) intrinsics correctly
On rust-lang/stdarch#1765, it has been pointed out that two RISC-V (64-bit only) intrinsics to perform AES key scheduling have wrong target feature.
`aes64ks1i` and `aes64ks2` instructions require *either* Zkne (scalar cryptography: AES encryption) or Zknd (scalar cryptography: AES decryption) extension (or both) but corresponding Rust intrinsics (in `core::arch::riscv64`) required *both* Zkne and Zknd extensions.
An excerpt from the original intrinsics:
```rust
#[target_feature(enable = "zkne", enable = "zknd")]
```
To fix that, we need to:
1. Represent a condition where *either* Zkne or Zknd is available and
2. Workaround an issue: `llvm.riscv.aes64ks1i` / `llvm.riscv.aes64ks2` LLVM intrinsics require either Zkne or Zknd extension.
This PR attempts to resolve them by:
1. Adding a perma-unstable RISC-V target feature: `zkne_or_zknd` (implied from both `zkne` and `zknd`) and
2. Using inline assembly to construct machine code directly (because `zkne_or_zknd` alone cannot imply neither Zkne nor Zknd, we cannot use LLVM intrinsics).
The author confirmed that we can construct an AES key scheduling function with decent performance using fixed `aes64ks1i` and `aes64ks2` intrinsics (with optimization enabled).
Jonathan Brouwer
dc103c4cd9