Fix race condition in Redirection global list using CAS.

runtime/vm/simulator_arm64.cc

Index: runtime/vm/simulator_arm64.cc
===================================================================
--- runtime/vm/simulator_arm64.cc	(revision 43532)
+++ runtime/vm/simulator_arm64.cc	(working copy)
@@ -850,7 +850,17 @@
         argument_count_(argument_count),
         hlt_instruction_(kRedirectInstruction),
         next_(list_) {
-    list_ = this;
+    // Atomically prepend this element to the front of the global list.
+    // Note: Since elements are never removed, there is no ABA issue.
+    Redirection* old_list = list_;
+    do {
+      next_ = old_list;
+      old_list = reinterpret_cast<Redirection*>(
+          AtomicOperations::CompareAndSwapWord(
+              reinterpret_cast<uword*>(&list_),
+              reinterpret_cast<uword>(next_),
+              reinterpret_cast<uword>(this)));
+    } while (old_list != next_);
   }
 
   uword external_function_;