Rollup merge of #118610 - krasimirgg:llvm-18-dec, r=nikic

update target feature following LLVM API change

LLVM commit e817966718 renamed* the `unaligned-scalar-mem` target feature to `fast-unaligned-access`.

(*) technically the commit folded two previous features into one, but there are no references to the other one in rust.

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -263,6 +263,10 @@ pub fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> LLVMFeature<'a> {
             "sve2-bitperm",
             TargetFeatureFoldStrength::EnableOnly("neon"),
         ),
+        // The unaligned-scalar-mem feature was renamed to fast-unaligned-access.
+        ("riscv32" | "riscv64", "fast-unaligned-access") if get_version().0 <= 17 => {
+            LLVMFeature::new("unaligned-scalar-mem")
+        }
         (_, s) => LLVMFeature::new(s),
     }
 }