v8a as default aarch64 target

After 8689f5e landed, LLVM takes the intersection of v8a and v8r as default.
This commit brings back v8a support by explicitly specifying v8a in the feature list.

This should solve #97724.

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -491,6 +491,11 @@ pub(crate) fn global_llvm_features(sess: &Session, diagnostics: bool) -> Vec<Str
         .flatten();
     features.extend(feats);
 
+    // FIXME: Move v8a to target definition list when earliest supported LLVM is 14.
+    if get_version() >= (14, 0, 0) && sess.target.arch == "aarch64" {
+        features.push("+v8a".into());
+    }
+
     if diagnostics && let Some(f) = check_tied_features(sess, &featsmap) {
         sess.emit_err(TargetFeatureDisableOrEnable {
             features: f,