[ARM64] Initial commit for arm64 backend

Based entirely off existing xbyak x86 implementation and available
tests. Still needs a lot of optimization and testing on non-Windows
platforms.

So far passes all tests and boots at least some games on Windows.

src/xenia/cpu/backend/a64/a64_sequences.h

@@ -0,0 +1,52 @@
+/**
+ ******************************************************************************
+ * Xenia : Xbox 360 Emulator Research Project                                 *
+ ******************************************************************************
+ * Copyright 2026 Ben Vanik. All rights reserved.                             *
+ * Released under the BSD license - see LICENSE in the root for more details. *
+ ******************************************************************************
+ */
+
+#ifndef XENIA_CPU_BACKEND_A64_A64_SEQUENCES_H_
+#define XENIA_CPU_BACKEND_A64_A64_SEQUENCES_H_
+
+#include <unordered_map>
+
+#include "xenia/cpu/hir/instr.h"
+
+namespace xe {
+namespace cpu {
+namespace backend {
+namespace a64 {
+
+class A64Emitter;
+
+typedef bool (*SequenceSelectFn)(A64Emitter&, const hir::Instr*, uint32_t ikey);
+extern std::unordered_map<uint32_t, SequenceSelectFn> sequence_table;
+
+template <typename T>
+bool Register() {
+  sequence_table.insert({T::head_key(), T::Select});
+  return true;
+}
+
+template <typename T, typename Tn, typename... Ts>
+static bool Register() {
+  bool b = true;
+  b = b && Register<T>();
+  b = b && Register<Tn, Ts...>();
+  return b;
+}
+
+#define EMITTER_OPCODE_TABLE(name, ...) \
+  const auto A64_INSTR_##name = Register<__VA_ARGS__>();
+
+bool SelectSequence(A64Emitter* e, const hir::Instr* i,
+                    const hir::Instr** new_tail);
+
+}  // namespace a64
+}  // namespace backend
+}  // namespace cpu
+}  // namespace xe
+
+#endif  // XENIA_CPU_BACKEND_A64_A64_SEQUENCES_H_