[DO NOT MERGE] update target feature following LLVM API change

LLVM commit llvm/llvm-project@e817966
renamed the `unaligned-scalar-mem` target feature to `fast-unaligned-access`.

Author: krasimirgg

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -291,9 +291,9 @@ const RISCV_ALLOWED_FEATURES: &[(&str, Stability)] = &[
     ("d", Unstable(sym::riscv_target_feature)),
     ("e", Unstable(sym::riscv_target_feature)),
     ("f", Unstable(sym::riscv_target_feature)),
+    ("fast-unaligned-access", Unstable(sym::riscv_target_feature)),
     ("m", Stable),
     ("relax", Unstable(sym::riscv_target_feature)),
-    ("unaligned-scalar-mem", Unstable(sym::riscv_target_feature)),
     ("v", Unstable(sym::riscv_target_feature)),
     ("zba", Stable),
     ("zbb", Stable),