RISC-V: Add virtual target feature: zkne_or_zknd

Because some AES key scheduling instructions require *either* Zkne or
Zknd extension, we must have a target feature to represent
`(Zkne || Zknd)`.

This commit adds (perma-unstable) target feature to the RISC-V
architecture: `zkne_or_zknd` for this purpose.

Helped-by: sayantn <sayantn05@gmail.com>

Author: a4lg

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -251,6 +251,8 @@ pub(crate) fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> Option<LLVMFea
         // Filter out features that are not supported by the current LLVM version
         ("loongarch32" | "loongarch64", "32s") if major < 21 => None,
         ("powerpc", "power8-crypto") => Some(LLVMFeature::new("crypto")),
+        // Filter out Rust-specific *virtual* target features
+        ("riscv32" | "riscv64", "zkne_or_zknd") => None,
         ("sparc", "leoncasa") => Some(LLVMFeature::new("hasleoncasa")),
         ("x86", "sse4.2") => Some(LLVMFeature::with_dependencies(
             "sse4.2",

compiler/rustc_target/src/target_features.rs

@@ -691,8 +691,9 @@ static RISCV_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("zimop", Unstable(sym::riscv_target_feature), &[]),
     ("zk", Stable, &["zkn", "zkr", "zkt"]),
     ("zkn", Stable, &["zbkb", "zbkc", "zbkx", "zkne", "zknd", "zknh"]),
-    ("zknd", Stable, &[]),
-    ("zkne", Stable, &[]),
+    ("zknd", Stable, &["zkne_or_zknd"]),
+    ("zkne", Stable, &["zkne_or_zknd"]),
+    ("zkne_or_zknd", Unstable(sym::riscv_target_feature), &[]), // Not an extension
     ("zknh", Stable, &[]),
     ("zkr", Stable, &[]),
     ("zks", Stable, &["zbkb", "zbkc", "zbkx", "zksed", "zksh"]),

tests/ui/check-cfg/target_feature.stderr

@@ -424,6 +424,7 @@ LL |     cfg!(target_feature = "_UNEXPECTED_VALUE");
 `zkn`
 `zknd`
 `zkne`
+`zkne_or_zknd`
 `zknh`
 `zkr`
 `zks`