tests: accept some noise from LLVM 21 in symbols-all-mangled

I'm not entirely sure this is correct, but it doesn't feel
obviously-wrong so I figured I'd just start by sending a PR rather than
filing a bug and letting it linger.

@rustbot label llvm-main

Author: durin42

tests/run-make/symbols-all-mangled/rmake.rs

@@ -41,7 +41,13 @@ fn symbols_check_archive(path: &str) {
             continue; // Unfortunately LLVM doesn't allow us to mangle this symbol
         }
 
-        panic!("Unmangled symbol found: {name}");
+        if name.contains(".llvm.") {
+            // Starting in LLVM 21 we get various implementation-detail functions which
+            // contain .llvm. that are not a problem.
+            continue;
+        }
+
+        panic!("Unmangled symbol found in {path}: {name}");
     }
 }